[PATCH] add missed tests for gist index

Hi hackers,

Two test-only patches.

gist.sql vacuums only once after deleting rows, so the emptied leaf pages
are never recycled or reused, and no test deletes every entry. Nothing
replays a GiST page deletion through recovery either.

0001 adds those steps to gist.sql. 0002 adds a recovery test that crashes
and restarts the server after the same workload, with
wal_consistency_checking set. Both compare an index scan against a
sequential scan.

Coverage of src/backend/access/gist goes from 80.3% to 87.9%; the redo side
of gistxlog.c was entirely unexecuted before.

Thanks,
Shihao