test: fix test_pageserver_recovery flakyness

[VladLazar]

Problem

We recently introduced log file validation for the storage controller. The heartbeater will WARN when it fails
for a node, hence the test fails.

Closes #7159

Summary of changes

• Warn only once for each set of heartbeat retries
• Allow list heartbeat warns

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]

2718 tests run: 2580 passed, 0 failed, 138 skipped (full report)

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Flaky tests (1)

Postgres 16

• test_timeline_size_quota_on_startup: release

Code coverage* (full report)

• functions: 28.2% (6289 of 22336 functions)
• lines: 46.9% (44180 of 94150 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
cf1797b at 2024-03-23T18:15:24.885Z :recycle:}

[VladLazar]

120 runs of the test with no failures here. Seems stable.