This makes test suite happy again (at 44) while fixing the sequences
we were fixing, which were the following with KhmerUI.ttf:

  U+1789,U+17BC
  U+1789,U+17D2,U+1789
  U+1789,U+17D2,U+1789,U+17BC

Fixes rest of https://github.com/harfbuzz/harfbuzz/issues/974