The code was setting GlyphClassDef.classDefs for the base font to an
empty dict then reading it from all fonts. It accidentally works when
creating variable fonts because the GlyphClassDef of the other fonts
will be used, but when mutating there is only one font.
Fix by reading the glyph classes before assigning to an empty dict.
this adds three new tests for varLib.build:
1) the input designspace contains a single 'weight' axis with <map>
elements that modify the normalization mapping;
2) the input designspace contains two 'weight' and 'width' axes both
with <map> elements, but the 'width' axis only contains identity
mappings;
3) the input designspace contains two 'weight' and 'width' axes, but
only one axis ('weight') has some <map> elements.
The master interpolatable ttx files for 'TestFamily3' are a subset of
Noto Sans containing only the letters to type the word "FontTools".
These originated from TTFs that were built from the test UFO masters using fontmake v1.2.0 with the command `fontmake -m BuildTest.designspace -o variable`
