fix(pageserver): ensure test creates valid layer map

[skyzh]

Problem

I'd like to add some constraints to the layer map we generate in tests.

(1) is the layer map that the current compaction algorithm will produce. There is a property that for all delta layer, all delta layer overlaps with it on the LSN axis will have the same LSN range.
(2) is the layer map that cannot be produced with the legacy compaction algorithm.
(3) is the layer map that will be produced by the future tiered-compaction algorithm. The current validator does not allow that but we can modify the algorithm to allow it in the future.

Summary of changes

Add a validator to check if the layer map is valid and refactor the test cases to include delta layer start/end LSN.

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]

3006 tests run: 2891 passed, 0 failed, 115 skipped (full report)

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Code coverage* (full report)

• functions: 32.7% (6925 of 21208 functions)
• lines: 50.0% (54384 of 108664 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
d53c5ac at 2024-07-03T18:59:30.367Z :recycle:}

[problame]

Perhaps the create_test_timeline_with_layers should also take the key range so it can create arbitrarily shaped layers instead of using the minimal key range.

That would allow test code to "draw" arbitrary layer maps, subject to the validation logic in create_test_timeline_with_layers.

[skyzh]

refactored to have a type to describe a delta layer, so that we can specify lsn+key range

[problame]

Thanks for the new type!

The ASCII art is still not quite right.