Add MithrilFixture builder

[Alenar]

Content

This PR includes refactor the way to construct test signers by adding the MithrilFixtureBuilder that allow to generate them alongside a stake distribution.

Pre-submit checklist

• Branch
  • Tests are provided (if possible)
  • Commit sequence broadly makes sense
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Comments

This PR also reorganizes the test helpers in mithril common by moving the apispec & fake_data to the new test_utils module.
Also all mithril-common test helpers are now behind the test-only feature.

Issue(s)

Relates to #663

[jpraynaud]

LGTM 👍

[github-actions]

Test Results

    3 files  ±0    28 suites  ±0   3m 10s ⏱️ +9s
381 tests +4  381 ✔️ +4  0 💤 ±0  0 ❌ ±0 
442 runs  +4  442 ✔️ +4  0 💤 ±0  0 ❌ ±0 

Results for commit aecd616. ± Comparison against base commit 07b5e8b.

This pull request removes 3 and adds 7 tests. Note that renamed tests count towards both.

apispec::tests ‑ test_apispec_validate_errors
apispec::tests ‑ test_apispec_validate_ok
src/digesters/immutable_digester.rs - digesters::immutable_digester ‑ ImmutableDigester (line 13)

src/digesters/immutable_digester.rs - digesters::immutable_digester ‑ ImmutableDigester (line 12)
test_utils::apispec::tests ‑ test_apispec_validate_errors
test_utils::apispec::tests ‑ test_apispec_validate_ok
test_utils::fixture_builder::tests ‑ each_parties_generated_with_random_stake_distribution_have_different_stakes
test_utils::fixture_builder::tests ‑ random_stake_distribution_generates_as_many_signers_as_parties
test_utils::fixture_builder::tests ‑ with_protocol_params
test_utils::fixture_builder::tests ‑ with_signers