tests: increase approx size equal threshold to avoid test_lsn_lease_size flakiness

[yliang412]

Summary of changes

Increase the assert_size_approx_equal threshold to avoid flakiness of test_lsn_lease_size. Still needs more investigation to fully resolve #8293.

• Also set autovacuum=off for the endpoint we are running in the test.

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.
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Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

3042 tests run: 2927 passed, 0 failed, 115 skipped (full report)

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Code coverage* (full report)

• functions: 32.6% (6939 of 21280 functions)
• lines: 50.0% (54556 of 109068 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
cac6ee6 at 2024-07-08T20:06:12.801Z :recycle:}

[yliang412]

Followup: on PG16, it's much larger (allure).

[arpad-m]

good as a temporary workaround, later we can find a better fix