Don't run regcore_pgsql tests in every env.
Don't run regcore_pgsql tests in every env.

The pgsql tests require `regcore_pgsql` be one of the installed apps. As
we don't have that configuration in each test environment (at least, at the
moment), specify which packages to test based on whether or not pgsql is part
of the environment.

eregs/regulations-core,18F/regulations-core