Add some isolation tests for CLUSTER

Add some isolation tests for CLUSTER

This commit adds two isolation tests for CLUSTER, using:
- A normal table, making sure that CLUSTER blocks and completes if the
table is locked by a concurrent session.
- A partitioned table with a partition owned by a different user.  If
the partitioned table is locked by a concurrent session, CLUSTER on the
partitioned table should block.  If the partition owned by a different
user is locked, CLUSTER on its partitioned table should complete and
skip the partition.  3f19e17 has added an early check to ignore such a
partition with a SQL regression test, but this was not checking that
CLUSTER should not block.

Discussion: https://postgr.es/m/YlqveniXn9AI6RFZ@paquier.xyz