Minor moving around
Minor
Fix Mongolian Variation Selectors for fonts without GDEF

Originally we fixed those in 79d1007a501fd63c0ba4d51038c513e6b8b94740.
However, fonts like MongolianWhite don't have GDEF, but have IgnoreMarks
in their LigatureSubstitute init/etc features.  We were synthesizing a
GDEF class of mark for Mongolian Variation Selectors and as such the
ligature lookups where not matching.  Uniscribe doesn't do that.

I tried with more sophisticated fixes, like, if there is no GDEF and
a lookup-flag mismatch happens, instead of rejecting a match, try
skipping that glyph.  That surely produces some interesting behavior,
but since we don't want to support fonts missing GDEF more than we have
to, I went for this simpler fix which is to always mark
default-ignorables as base when synthesizing GDEF.

Micro-test added.

Fixes rest of https://bugs.freedesktop.org/show_bug.cgi?id=65258
[tests] Add record-test.sh