test(pageserver): add test wal record for unit testing

[skyzh]

Problem

#8002

Summary of changes

We need mock WAL record to make it easier to write unit tests. This pull request adds such a record. It has clear flag and append field. The tests for legacy-enhanced compaction are not modified yet and will be part of the next pull request.

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]

3210 tests run: 3068 passed, 0 failed, 142 skipped (full report)

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Code coverage* (full report)

• functions: 31.6% (6636 of 20995 functions)
• lines: 48.6% (51598 of 106148 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
d20086c at 2024-06-12T17:30:54.411Z :recycle:}

[problame]

The clear doesn't seem to be used in tests?

Also, have you thought about another field that controls whether the record is will_init?

Approving since this is cfg(test) only and easily changeable later.

[skyzh]

that's true, will add more tests before merging

[skyzh]

There are some limitations with NeonWalRecords:

                let base_img = base_img.expect("Neon WAL redo requires base image").1;

I added the variants for clear/init records but did not use them in the test cases because of the above limitation. Will have a separate pull request to discuss how to deal with it. Besides that, if there are no further comments, I will get this pull request merged tomorrow.