[subset] add regression test for MATH closure construction fix

A glyph pulled into the subset only by MATH variant closure must not
keep its own MathGlyphConstruction. Closure is a single pass, so the
variants such a construction references are not themselves closed over,
and keeping it leaves dangling references to glyphs absent from the
subset. The existing MATH test fonts never place a size variant in its
own coverage, so they cannot exercise this; this test builds a font
that does.