pgsql: Fix unstable tests in partition_merge.sql and partition_split.sq

Fix unstable tests in partition_merge.sql and partition_split.sql.

The tests added by commit c086896625 were unstable due to
missing schema names when checking pg_tables and pg_indexes.

Backpatch to v17.

Reported by buildfarm.

Branch
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REL_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6a700da46e6279a0df6a996fd19c5aabdf7a4b56

Modified Files
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src/test/regress/expected/partition_merge.out | 7 +++++--
src/test/regress/expected/partition_split.out | 6 ++++--
src/test/regress/sql/partition_merge.sql      | 7 +++++--
src/test/regress/sql/partition_split.sql      | 6 ++++--
4 files changed, 18 insertions(+), 8 deletions(-)