Add a test mode to mount.set_fstab

This allows the fstab to be checked to see if changes will be made. This
does not change the return value of this function in any way, but just
prevents the fstab from being rewritten if test=True.
Fix regression in mount.mounted state, improve reporting

This fixes the regression from #6545. In addition, running this state in
test mode now properly identifies if the fstab entry exists but needs to
be modified, and reports this in the state output.
Merge pull request #6566 from terminalmage/issue6545

Fix regression in mount.mounted state