monitor: Add foreach output helper and fix foreach crtc helper

The foreach CRTC monitor mode helper incorrectly iterated over outputs
without CRTC when non-tiled modes were set on tiled monitors. This was
not expected by callers, so fix the helper to only iterate over active
outputs (that has or should have a CRTC).

The test cases uses the incorrect behaviour of the foreach CRTC helper
to check that the disabled outputs mode are set to NULL, so add a
foreach output helper and change the tests to use that instead.

https://bugzilla.gnome.org/show_bug.cgi?id=730551