Move not_graphql_context_test_suite to root tox file
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.
Test Plan: bk
Reviewers: dgibson, alangenfeld
Reviewed By: alangenfeld
Differential Revision: https://dagster.phacility.com/D5956