Make sepgsql regression tests robust vs. collation differences

Make sepgsql regression tests robust vs. collation differences

In commit 25542d77, regression test coverage was added to sepgsql
for partitioned tables. Unfortunately it was not robust in the face
of collation differences, per the buildfarm. Force "C" collation
in order to fix that.

Discussion: https://postgr.es/m/flat/623bcaae-112e-ced0-8c22-a84f75ae0c53%40joeconway.com