In #2356, we assume that page_files are not empty. However, unit tests randomly crash after that.

1. A global RegionPersister created with path "./tmp/kvstore" and register a GC task on BackgroundService
2. Before some unit test of the storage layer run, they would clean up old files on disk. However, we directly remove the whole "./tmp" for convenience in some tests.
3. The GC task of RegionPersister run, find out that there are no page files on "./tmp/kvstore", and the unit test crash.

To fix this problem

1. Add check for PageStorage::gc to avoid crash Add length check while running PageStorage GC #2394
2. Refine the unit tests that remove the whole "./tmp" @jiaqizho

For the second task:

One of those tests is the Segment_test
https://github.com/pingcap/tics/blob/9f03c0570c337cbc1d7d72fee8680e84231d8e9d/dbms/src/Storages/DeltaMerge/tests/gtest_dm_segment.cpp#L43-L48

The real paths of this test is generated by the StoragePathPool object (for multi-disk deployment)
https://github.com/pingcap/tics/blob/9f03c0570c337cbc1d7d72fee8680e84231d8e9d/dbms/src/Storages/DeltaMerge/tests/gtest_dm_segment.cpp#L71-L73

We should find a more elegant way to clean up old files for unit tests, instead of removing "./tmp" brutally. Hope that there would be no overlap between different test suite.
For example, each test case under the test suite Segment_test store its data under "./tmp/Segment_test" while each test case under the test suite Segment_test_2 store its data under "./tmp/Segment_Common_Handle_test". Before running each test case for Segment_test, it only removes the old files under "./tmp/Segment_test".

(After that, we can run different test suites parallelly one day. not included by this issue).