gitlab-ci: use a test-specific tagset on Windows

Some machines have trouble running OpenGL for whatever reason, so let
them do builds and test everywhere else.
6 jobs for ci-windows-test-machines in 0 seconds (queued for 1 second)
Status Name Job ID Coverage
  External
passed cdash-commit

passed ghostflow-branch-check-master

passed ghostflow-branch-check-release

passed cdash-commit

passed cdash-commit

passed cdash-commit