Update what_tests_can_be_run.md - add admin username/password environment variables
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21657 adds GITLAB_ADMIN_USERNAME
and GITLAB_ADMIN_PASSWORD
environment variables that can be used to override the default GDK values that are used when adding a license.
Updating the docs to include the new environment variables.
This is related to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21354