fix(test suite): some tests leak child processes

[problame]

Epic: #6485

Before this PR, some tests would leak child processes.
We found them using the approach in
#6470.

This PR fixes the findings because PR#6470 is being delayed due to
security concerns.

[github-actions]

2340 tests run: 2247 passed, 0 failed, 93 skipped (full report)

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Flaky tests (2)

Postgres 16

• test_statvfs_pressure_min_avail_bytes[tokio-epoll-uring]: debug

Postgres 15

• test_pageserver_recovery[tokio-epoll-uring]: release

Code coverage (full report)

• functions: 54.2% (10851 of 20011 functions)
• lines: 81.1% (61449 of 75744 lines)

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_{The comment gets automatically updated with the latest test results
7927c67 at 2024-01-26T18:33:38.555Z :recycle:}