3ca43dbbb6 adds regression tests with permission checks. The conflict has
been observed at buildfarm member piculet.
This commit fixes the conflict in the following way.
1. partition_split.sql now uses role names regress_partition_split_alice and
regress_partition_split_bob (it mistakenly used
regress_partition_merge_alice and regress_partition_merge_bob before).
2. Permissions on schemas partitions_merge_schema and partition_split_schema
are granted to corresponding roles. Before, the lack of permissions led to
the creation of objects in the public schema and potential conflict.
Reported-by: Daniel Gustafsson
Discussion: https://postgr.es/m/03A07EF6-98D2-419B-A3AA-A111C64CC207%40yesql.se