Simple glyph composition feature tests

Tone mark modifier characters in Unicode (U+02E*) often use the 'ccmp' OpenType feature to combine marks:

˩ ˥ ˩˥ (˩˥)

˥ ˩ ˥˩ (˥˩)

˦ ˥ ˦˥ (˦˥)

˥ ˦ ˥˦ (˥˦)

˧ ˥ ˧˥ (˧˥)

˥ ˧ ˥˧ (˥˧)