feat: backend selective testing

[joshuarli]

improved version of #105500

also you can test test selection and dynamic shard count locally with something like COVERAGE_DB_PATH=~/Downloads/.coverage.combined GITHUB_PR_BASE_REF=a237e1e8676f5ed76a2efde1b649e9aecf85fe52 python scripts/select-tests.py

• test selection is now done entirely at dynamic sharding step and in a self contained script that doesn't require setup-sentry
• the selected list of test nodeids is then passed directly to the per-shard pytest invocation which then doesn't need to collect the full test suite at all!
  • pytest collectitems needs no modification since we select the whole list of tests first then just pass it directly to pytest.main!
• performance improvements
  • changed-files is now just a git call
  • replaced many individual LIKE sqlite queries with a single IN call - way faster

[rbro112]

Working on the main branch upload and I can handle swapping this for Gcloud download in the future

[rbro112]

I think we'll need to keep this around for the stock backend-test workflow. Might be best to leave this in and we can leverage this for both backend.yml and backend_selective_testing - in this case we can pass the selected_tests from the select-tests.py as an arg to this, or just a None/empty array if backend.yml (all tests)

[joshuarli]

i was thinking we don't need dynamic sharding for the old backend workflow

we need to hardcode it back to 22 in main and remove this in a separate pr

[getsantry]

This issue has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you remove the label Waiting for: Community, I will leave it alone ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀