FIX read_fiducials():  set points' coord_frame

I noticed that read_tag() sets the coord_frame to 0 by default, which is not true (it should be the coord_frame stored separately in the fiducials file. However this PR breaks the tests since the file it loads is not actually a proper fiducials file. Would it be ok to include the file that comes with mne (`share/mne/mne_analyze/fsaverage/fsaverage-fiducials.fif`) with the test data?