test: less flaky test_synthetic_size_while_deleting

[koivunej]

#7585 introduced test case for deletions while synthetic size is being calculated. The test has a race against deletion, but we only accept one outcome. Fix it to accept 404 as well, as we cannot control from outside which outcome happens.

Evidence: https://neon-github-public-dev.s3.amazonaws.com/reports/pr-7456/8970595458/index.html#/testresult/32a5b2f8c4094bdb

[koivunej]

Disabled auto-merge while searching if we need an additional allowed_error, but the 404 is logged at INFO, so no need.

[github-actions]

2904 tests run: 2783 passed, 0 failed, 121 skipped (full report)

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Flaky tests (3)

Postgres 16

• test_delete_tenant_exercise_crash_safety_failpoints[real_s3-timeline-delete-before-index-deleted-at-False-Check.RETRY_WITHOUT_RESTART]: release
• test_vm_bit_clear_on_heap_lock: debug

Postgres 15

• test_gc_aggressive: debug

Code coverage* (full report)

• functions: 31.4% (6237 of 19864 functions)
• lines: 47.1% (46732 of 99252 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
152f1f1 at 2024-05-06T16:00:17.103Z :recycle:}