Clean up per-test DB paths after unit tests to avoid ENOSPC

[xingbowang]

Summary

When we migrated from one test one process model to sharding model, we didn't add db folder cleanup. This causes db folders to pile up in unit test. Add db folder clean up after each test completion on success. This keep the disk utilization low.

• clean up paths registered through PerThreadDBPath() after each passing test so sharded unit-test runs do not accumulate leftover DB directories
• keep failed-test artifacts when KEEP_DB is set or the test fails, while still resetting SyncPoint state between tests
• add make check disk-usage diagnostics that report the largest per-test temp directories and DB-like directories on failure
• add Python coverage for the diagnostics helper and C++ coverage for registered path cleanup

Test Plan

• python3 build_tools/make_check_dbdir_report_test.py
• make -j14 testutil_test
• /usr/local/bin/timeout 60 ./testutil_test --gtest_filter="TestUtil.CleanupRegisteredPerTestPathsRemovesDirectory:TestUtil.CleanupRegisteredPerTestPathsRemovesFile"

[github-actions]

⚠️ clang-tidy: 1 warning(s) on changed lines

Completed in 29.0s.

Summary by check

Check	Count
concurrency-mt-unsafe	1
Total	1

Details

test_util/testharness.cc (1 warning(s))

test_util/testharness.cc:122:9: warning: function is not thread safe [concurrency-mt-unsafe]

[meta-codesync]

@xingbowang has imported this pull request. If you are a Meta employee, you can view this in D101068798.

[meta-codesync]

@xingbowang has imported this pull request. If you are a Meta employee, you can view this in D101068798.