GitLab

- updated `tests/TESTING.md`
- cgroup, snapshotter, and install tests all under tests/vagrant
- cgroup and snapshotter workflows trigger for all code changes on all branches (excluding markdown docs, install script, and other vagrant tests)
- install workflow triggers for relevant script and fixture changes, only on master or pull-requests that target it
- integration and unit test workflows should not trigger for install script changes nor anything under tests/vagrant
Signed-off-by: Jacob Blain Christen <jacob@rancher.com>