Disable admin tests by default

We should be clear about when we run admin tests in a particular environment.

Right now we're not running the full suite of tests on Canary or Prod, but if we did all the tests that require an admin user would fail.

This will disable those tests (at least the ones that are tagged with :requires_admin. Tagging remaining tests here: gitlab-org/gitlab!19926 (merged)).

We do have an admin user on Staging, so we can set QA_CAN_TEST_ADMIN_FEATURES to true for those tests: staging!41 (merged)