[google fonts] New check: all default fallback positions are registered

[RosaWagner]

google/fonts#2859 (comment)

We have to make sure that the every default positions of fonts is registered in the axis registry. I think it would be a FAIL otherwise (according to the internal issues we had because of this).

Reasons:

1. For the ZIP file to be accurate — otherwise it doesn't generate the chosen default font. For eg., Newsreader has a default value on the opsz axis of 16pt. If 16pt is not a registered fallback position in the opticalsize.textproto, then the Zip file contain another position as default (like 14pt).
2. For the Variable font to display the correct location on the specimen page.
3. For VF with no weight axis to be just… displayed. For eg. Ballet, which has no weight axis, was not appearing in sandbox because default position on opsz axis was 16pt, and it was not a registered fallback positon.