Refs #9205 - While we're revisiting the property, add a validator.

(This change is a safe one to make as the property was essentially
mandatory anyway -- previously we were just checking in the `exec`
portion of the alg rather than in `init`.)
Refs #9205 - Change existing unit tests to use ScopedFiles.
Refs #9205 - Allow loading of masks by IDF as well as by name.
Refs #9205 - Remove commented-out line.
Merge remote-tracking branch 'origin/feature/9205_extend_instrument_prop_of_loadmask_to_accept_idfs'