Make the compaction warning more tolerant

[arpad-m]

Problem

The performance benchmark in test_runner/performance/test_layer_map.py is currently failing due to the warning added in #4888.

Summary of changes

The test mentioned has a compaction_target_size of 8192, which is just one page size. This is an unattainable goal, as we generate at least three pages: one for the header, one for the b-tree (minimally sized ones have just the root node in a single page), one for the data.

Therefore, we add two pages to the warning limit. The warning text becomes a bit less accurate but I think this is okay.

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]

1608 tests run: 1533 passed, 0 failed, 75 skipped (full report)

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_{The comment gets automatically updated with the latest test results
415f49d at 2023-08-18T13:59:19.783Z :recycle:}

[koivunej]

I don't know if this is any better. Could we just allowed_error for the one test?

(2+2)*8192 for the test_layer_map.rs here: https://neon-github-public-dev.s3.amazonaws.com/reports/main/5889544346/index.html#suites/b8bb9797b578168beaf6275d4a5cca96/a842de44855768cd

[arpad-m]

I don't really like allowing tests that use bad config parameters. Instead, I prefer either disallowing those bad config parameters via an error, or silently fixing them (like what my PR proposes). Could we talk about the details in a google call? I've sent you an e-mail.

[arpad-m]

Removed the part where I reduced the potential overshoot. Personally I still think that's a good idea, but it isn't as pressing as fixing the warning.