test_compatibility: flush in the end

[koivunej]

test_forward_compatibility is still often failing at graceful shutdown. Fix this by explicit flush before shutdown.

Example: https://neon-github-public-dev.s3.amazonaws.com/reports/main/10506613738/index.html#testresult/5e7111907f7ecfb2/

Cc: #8655 and #8708
Previous attempt: #8787

[bayandin]

Nice!

[github-actions]

3801 tests run: 3695 passed, 0 failed, 106 skipped (full report)

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Flaky tests (2)

Postgres 16

• test_subscriber_synchronous_commit: release
• test_subscriber_restart: debug

Code coverage* (full report)

• functions: 32.4% (7237 of 22342 functions)
• lines: 50.4% (58528 of 116127 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
1fa3f89 at 2024-08-22T15:40:24.592Z :recycle:}