Stupid logic error in build_stat

[simoncozens]

Found while porting. I can't think of a way to trigger this, but it just looks wrong:

https://github.com/googlefonts/axisregistry/blob/3db2ff673bb85c8ecee55f9e38ce601e8bc2e2f4/Lib/axisregistry/init.py#L229-L249

Suppose an axis is in fallbacks_in_names but not in fallbacks_in_fvar; it gets added during the first loop above. But suppose it is also in fallbacks_in_siblings. Because we didn't do a seen_axes.add(axis) during the fallbacks_in_names loop, it gets added twice.

[simoncozens]

In fact, I don't need a way to trigger it, we explicitly test for the axis being added twice.

https://github.com/googlefonts/axisregistry/blob/3db2ff673bb85c8ecee55f9e38ce601e8bc2e2f4/tests/data/OpenSans-Italic%5Bwdth%2Cwght%5D_STAT.ttx#L15-L24