[PATCH] tests: verify renamed index functionality in alter_table

Hello,

The alter_table test has a FIXME saying the index rename test should verify
the index still works afterward. Currently it only checks that ALTER INDEX
RENAME succeeds, not that the renamed index is usable.

-- ALTER TABLE ... RENAME on non-table relations

-- renaming indexes (FIXME: this should probably test the index's
> functionality)

This patch adds a simple verification: EXPLAIN to show the planner finds
the index, and a SELECT to confirm it returns correct data. Uses the
standard enable_seqscan/bitmapscan/indexonlyscan=off pattern to force a
stable Index Scan across environments.

Test-only change. Verified with:
- make check TESTS=alter_table # passes
- make check # all 236 tests pass

Thanks,
Dharin