The coverage threshold (0.5 in fbd4e5ad1, lowered to 0.3 in d4106ef2d to
get CI green) is environment-dependent: a correctly sized emoji covers
~0.84 of its cell on one box but ~0.39 in CI, while the buggy render is
~0.28, so the margin is thin and font/cell dependent.
Render the same font two ways at one size instead: via its fontconfig
descriptor (which carries the size-fixup matrix) and via its file path
(which does not). They must come out the same size; the bug shrinks the
descriptor one. Only the matrix differs, so the check no longer depends
on the environment or the emoji artwork.