Excess quoting in test: changes meaning of search_path

[nmisch]

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.