fix(pageserver): more flexible layer size test

[skyzh]

Problem

M-series macOS has different alignments/size for some fields (which I did not investigate in detail) and therefore this test cannot pass on macOS. Fixed by using <= for the comparison so that we do not test for an exact match.

observed by @yliang412

Summary of changes

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]

3204 tests run: 3065 passed, 0 failed, 139 skipped (full report)

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Flaky tests (5)

Postgres 16

• test_subscriber_restart: release
• test_compaction_induced_by_detaches_in_history: release
• test_vm_bit_clear_on_heap_lock: debug

Postgres 15

• test_statvfs_pressure_usage: debug
• test_vm_bit_clear_on_heap_lock: debug

Code coverage* (full report)

• functions: 31.6% (6607 of 20917 functions)
• lines: 48.5% (51065 of 105214 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
3b228fc at 2024-06-06T15:22:55.821Z :recycle:}

[skyzh]

Checked the test is running in CI while ignored on macOS.

        PASS [   0.007s] pageserver tenant::storage_layer::layer::tests::layer_size

Ready for review.