pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/21a10368eb3fce73f146d7e48b4d81496f60d965

Modified Files
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.../expected/cluster-conflict-partition.out        | 35 ++++++++++++++++++++
src/test/isolation/expected/cluster-conflict.out   | 19 +++++++++++
src/test/isolation/isolation_schedule              |  2 ++
.../specs/cluster-conflict-partition.spec          | 37 ++++++++++++++++++++++
src/test/isolation/specs/cluster-conflict.spec     | 30 ++++++++++++++++++
5 files changed, 123 insertions(+)