chore(docs): Add pointers to tests

TESTING.md

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+# Testing Noir
+
+The following is a list of locations where we can find tests, and the ways to run them.
+
+## Lexing and parsing
+
+The lexer unit tests can be found in [lexer.rs](compiler/noirc_frontend/src/lexer/lexer.rs), while the parser unit tests are spread out under the [parser module](compiler/noirc_frontend/src/parser/parser).
+
+## Frontend
+
+The frontend has [common compilation tests](compiler/noirc_frontend/src/tests.rs) as well as [tests per language feature](compiler/noirc_frontend/src/tests).
+
+## Comptime
+
+The [comptime tests](compiler/noirc_frontend/src/hir/comptime/tests.rs) include snippets of Noir code executed via interpreter.
+
+## Formatter
+
+Tests for `nargo fmt` include expected [inputs and outputs](tooling/nargo_fmt/tests), with test cases generated in [build.rs](tooling/nargo_fmt/build.rs) and executed in [execute.rs](tooling/nargo_fmt/tests/execute.rs).
+
+## Noir `stdlib`
+
+Any `#[test]` included in Noir's `std` library is automatically executed by [stdlib-tests.rs](tooling/nargo_cli/tests/stdlib-tests.rs).
+
+Some functions in `std` are covered by property based tests in [stdlib-props.rs](tooling/nargo_cli/tests/stdlib-props.rs), which compare the execution of Noir snippets them against Rust equivalents.
+
+Tests for a specific Noir library can be executed using some extended CLI options. For example the following command runs the tests in the `array` module:
+
+```shell
+cargo test -p nargo_cli --test stdlib-tests -- run_stdlib_tests array
+```
+
+Tests are executed with multiple compilation options, so there will be multiple groups of results about the same tests.
+
+## Integration tests
+
+The [test_programs](test_programs) directory contains sub-directories with small Noir projects, organized according to expected outcome.
+
+For example the programs in [execution_success](test_programs/execution_success) are expected to succeed with `nargo execute` and return a result. These programs have a `Prover.toml` file with the inputs expected by the circuit in the project's `main.rs` file. If the `Prover.toml` file has a `return` item, it will be compared to the value returned by the program.
+
+By contrast the `compile_success_*` variants are run with `nargo compile` and require no input files.
+
+There are two kinds of projects:
+* some cover a specific feature, e.g. [break_and_continue](/Users/aakoshh/Work/aztec/noir/test_programs/execution_success/break_and_continue)
+* others are regression test for a specific bug ticket, in which case their name is typically `regression_<ticket-number>`, or `<feature>_regression_<ticket-number>`.
+
+Similar to the format tests, integration tests cases are generated in [build.rs](tooling/nargo_cli/build.rs) and executed by [execute.rs](tooling/nargo_cli/tests/execute.rs), under the `nargo_cli` crate.
+
+The following command executes all integration tests:
+
+```shell
+cargo test -p nargo_cli --test execute
+```
+
+Similar to the `stdlib` tests, some appear multiple times due to being executed with different compilation options. To only execute them once, by the default configuration, we can use the following variant:
+
+```shell
+cargo test -p nargo_cli --test execute forcebrillig_false_inliner_i64_min
+```
+
+A specific test can be similarly executed by name, for example the following command executes all projects that include `sha256` in their name:
+
+```shell
+cargo test -p nargo_cli --test execute sha256
+```
+
+When new tests are added, their output needs to be checked in using [cargo insta](https://crates.io/crates/cargo-insta), so any change in return values, bytecode or printed output causes a test failure until accepted.
+
+## SSA passes
+
+The SSA passes can be found in the [opt](compiler/noirc_evaluator/src/ssa/opt) module. Each module contains unit tests specific to their pass.
+
+We can use the `--show-ssa` CLI option to print the SSA after each pass, or the `--show-ssa-pass` CLI option to limit the output to a specific passes. For example the following command would show SSA passes with either "Defunct" or "Simple" in their labels:
+
+```console
+$ cargo run -q -p nargo_cli -- compile --silence-warnings --force --show-ssa-pass Simple --show-ssa-pass Defunct
+After Defunctionalization (1):
+...
+
+After Inlining simple functions (1):
+brillig(inline) fn main f0 {
+...
+```
+
+The list of passes in the default pipeline can be found in [ssa.rs](compiler/noirc_evaluator/src/ssa.rs), along with their labels. Some of them appear multiple times; the number in parentheses helps differentiate between repeated executions of the same pass.
+
+There can be integration and regression tests which cover some of the SSA passes. To help find out which, we can use the [ssa_pass_impact.rs](tooling/nargo_cli/examples/ssa_pass_impact.rs) tool, which lists the tests under `execution_success` that have the most dramatic change during the passes matching the `--ssa-pass` label:
+
+```shell
+cargo run -p nargo_cli --example ssa_pass_impact -- --ssa-pass "Removing Unreachable Functions"
+```
+
+## Fuzz tests
+
+The SSA passes, ACIR and Brillig are covered by [fuzz tests](tooling/ast_fuzzer/README.md).
+
+The following commands executes the fuzz targets for a limited amount of time to get quick feedback during CI and development:
+
+```shell
+cargo test -p noir_ast_fuzzer --test smoke
+cargo test -p noir_ast_fuzzer_fuzz arbtest
+```
+
+Should it find any problems, the test would print a _seed_ which we can use to replicate the test, for example:
+
+```shell
+NOIR_ARBTEST_SEED=0x6819c61400001000 cargo test -p noir_ast_fuzzer_fuzz comptime_vs_brillig
+```
+
+When the `NOIR_ARBTEST_SEED` is present, or when there is a non-panicky error, the tests will print out the Noir AST and the `Prover.toml` file which can be used to reproduce the problem as an integration test.