Previously, all the tests that were not part of the dagster-graphql context test matrix were included in the `postgres_tox.ini` file and run as a part of the `dagster-graphql-postgres` BK step. This was due to one test: `test_readonly_variants`, which had a `dagster_postgres` dependency.
This diff updates the tox files to run all tests not part of the test matrix as a part of the `dagster-graphql` step, and adds a `variant` mark that only tests the `test_readonly_variants` in the `dagster-graphql-postgres` step.