fix(test): only test num of L1 layers in compaction smoke test

[skyzh]

Problem

close #9160

For whatever reason, pg17's WAL pattern seems different from others, which triggers some flaky behavior within the compaction smoke test.

Summary of changes

• Run L0 compaction before proceeding with the read benchmark.
• So that we can ensure the num of L0 layers is 0 and test the compaction behavior only with L1 layers.

We have a threshold for triggering L0 compaction. In some cases, the test case did not produce enough L0 layers to do a L0 compaction, therefore leaving the layer map with 3+ L0 layers above the L1 layers. This increases the average read depth for the timeline.

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]

5022 tests run: 4864 passed, 0 failed, 158 skipped (full report)

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Flaky tests (2)

Postgres 17

• test_pg_regress[None]: debug-x86-64

Postgres 16

• test_lr_with_slow_safekeeper: release-arm64

Code coverage* (full report)

• functions: 31.4% (7489 of 23887 functions)
• lines: 49.6% (60127 of 121270 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
2a24091 at 2024-10-02T14:35:45.312Z :recycle:}

[VladLazar]

Few comments, but looks good to me.