Fix regression tests conflict in 3ca43dbbb6

Fix regression tests conflict in 3ca43dbbb6

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