test(pageserver): add delta records tests for gc-compaction

[skyzh]

Problem

Part of #8002

This pull request adds tests for bottom-most gc-compaction with delta records. Also fixed a bug in the compaction process that creates overlapping delta layers by force splitting at the original delta layer boundary.

Summary of changes

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]

2941 tests run: 2824 passed, 0 failed, 117 skipped (full report)

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Flaky tests (1)

Postgres 15

• test_pageserver_chaos[None]: release

Code coverage* (full report)

• functions: 32.6% (6884 of 21137 functions)
• lines: 50.1% (53672 of 107040 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
b139c89 at 2024-06-24T15:55:07.173Z :recycle:}

[problame]

Test changes LGTM

Were the changes in compaction.rs exposed by the added tests?

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I don't fully understand why the split point business is necessary. Happy to follow up on this post-merge though.

[skyzh]

Were the changes in compaction.rs exposed by the added tests?

Yes, the tests reveal some bugs within the compaction algorithm. When it comes to the k-merge re-implementation of that part, we can do a thorough review.