pageserver: even more debug for test_secondary_downloads

[jcsp]

The latest failures of test_secondary_downloads are spooky: layers are missing on disk according to the test, but present according to the pageserver logs:

• Make the pageserver assert that layers are really present on disk and log the full path (debug mode only)
• Make the test dump a full listing on failure of the assert that failed the last two times

Related: #6966

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]

2748 tests run: 2609 passed, 0 failed, 139 skipped (full report)

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Code coverage* (full report)

• functions: 28.3% (6380 of 22554 functions)
• lines: 47.2% (44982 of 95310 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
4b56dd3 at 2024-04-03T10:16:27.163Z :recycle:}