test(pageserver): add test_gc_feedback_with_snapshots

[skyzh]

Problem

should be working after #8328 gets merged. Part of #8002

Summary of changes

adds a new perf benchmark case that ensures garbages can be collected with branches

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

3150 tests run: 3029 passed, 0 failed, 121 skipped (full report)

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Flaky tests (1)

Postgres 14

• test_partial_evict_tenant[relative_spare]: release

Code coverage* (full report)

• functions: 32.5% (7023 of 21597 functions)
• lines: 49.9% (55895 of 111995 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
d06d6c3 at 2024-07-30T20:19:15.723Z :recycle:}

[skyzh]

The benchmark is running in the benchmarks(3) job, while the original one is in benchmarks(2), seems that the CI job is very smart at splitting those things...