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.

tox.ini

@@ -19,7 +19,9 @@ extras =
   elastic: backend-elastic
   haystack: backend-haystack
   pgsql: backend-pgsql
-commands = pytest --cov --cov-append
+commands =
+  pytest --cov --cov-append regcore regcore_write regcore_read
+  pgsql: pytest --cov --cov-append regcore regcore_write regcore_read regcore_pgsql
 setenv =
   elastic: DJANGO_SETTINGS_MODULE = regcore.settings.elastic
   pgsql: DJANGO_SETTINGS_MODULE = regcore.settings.pgsql