chore: add unit tests for brillig-gen

[guipublic]

Description

Problem*

Related to #9799

Summary*

Unit tests for Brillig-gen

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Execution Time'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: b5a5ebb	Previous: b5ece4a	Ratio
sha512-100-bytes	0.081 s	0.059 s	1.37

This comment was automatically generated by workflow using github-action-benchmark.

CC: @TomAFrench

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Test Suite Duration'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: 813e76a	Previous: b5ece4a	Ratio
test_report_zkpassport_noir-ecdsa_	3 s	2 s	1.50

This comment was automatically generated by workflow using github-action-benchmark.

CC: @TomAFrench

[vezenovm]

Generally it is a bit challenging following the tests when they are all built to a single and final entry point artifact. I think using ssa_to_acir_program is good for testing a complete Brillig entry point (e.g., did we set up the call data entry point correctly, did we link our separate brillig artifacts together in the case of multiple procedures or functions). However, it clutters the tests when used for every unit test which is just looking at test the code gen of individual instructions or single functions. For example, I see for the brillig_add tests we have comments describing how each of the artifacts links together to form the entry point and the code gen result of the individual instruction we want to test. It is difficult for a new contributor to see exactly what is being tested without these types of comments on every test. Also, if we ever change the entry point logic we would have to update every Brillig gen unit test. We still want entry point tests but they should be their own thing.

We should make a display for BrilligArtifact and check snapshots against that for individual instruction unit tests. We don't need a separate parser or anything as we will just be asserting against a display. I'll make a follow-up to show what I am thinking.

EDIT: #10165