Excess quoting in test: changes meaning of search_path

See https://github.com/pgaudit/pgaudit/pull/260 for a report against another
project using syntax `SET search_path = 'pg_catalog, pg_temp'`. A search of
PGXN for `'pg_catalog,` turned up one match in a citus test. While this is
benign in a testing context, I recommend making a change like that pull
request made. This would be a security vulnerability if used in non-test,
production code. Correcting the test code reduces the chance of someone
copying a bad pattern into non-test code.