This is what Microsoft's implementation does. Marks that need advance
need to add it back using 'dist' or other feature in GPOS. Update tests to
match.
The new test suite runs tests included under
hb/test/shaping/tests/*.tests, which themselves reference
font files stored by sha1sum under hb/test/shaping/fonts/sha1sum.
The fonts are produced using a subsetter to only include glyphs
needed to run the test.
Four initial tests are added for (Chain)Context matching,
of which three currently fail.
