cdbappendonlystorage_test.c:
Test that calling appendonly_redo with XLOG_APPENDONLY_INSERT or
XLOG_APPENDONLY_TRUNCATE will call ao_insert_replay or
ao_truncate_replay respectively.

xlog_mm_test.c:
Test that an MMXLOG_REMOVE_FILE AO transaction log record will call
XLogAODropSegmentFile.

cdbmirroredappendonly_test.c:
Test that ao_insert_replay or ao_truncate_replay will call
XLogAOSegmentFile when we cannot find the AO segment file.

xlogutils_test.c:
Test that the invalid_page_tab hash table is properly used when
invalid AO segment files are encountered.

These four pieces combined will test the case when AO XLOG records are
replayed but the corresponding segment files are missing from
filesystem. The replay is entered into the invalid_page_tab hash table
until a corresponding MMXLOG_REMOVE_FILE XLOG record type comes along
to remove the entry.

Currently these unit tests are under --enable-segwalrep configure flag
in the Makefile but can be moved to the actual TARGET if non-segwalrep
tests are added and need to be tested.

Reference:
https://github.com/greenplum-db/gpdb/commit/b659d0479fa57c383e7dbe15043cd2ca9d46f77f