The wasmer-integration-tests-cli test suite isn't hygienic

[Michael-F-Bryan]

Motivation

Almost all of the tests in the wasmer-integration-tests-cli test suite are non-hygienic. They all use the same ~/.wasmer/ directory, which means that

1. Integration tests need to be run sequentially (i.e. --test-threads=1) and a failure midway through can sometimes mess things up for subsequent tests (I've noticed caching is particularly bad for this)
2. After the integration tests are done, the global ~/.wasmer/wasmer.toml file will be left in a broken state and you'll need to find your old access token and log in again

Proposed solution

The way we solve this problem is by making sure each integration test uses its own independent wasmer directory. We can do this fairly trivially by setting the $WASMER_DIR environment variable to something unique like $CARGO_TARGET_TMPDIR/wasmer-integration-tests-cli/$test_name

Cargo always sets provides integration tests with a $CARGO_TARGET_TMPDIR directory inside target/ they can use as a scratch space. That also means CI will get caching for free.