[test] Add generated tests for emoji clusters

Fixes https://github.com/harfbuzz/harfbuzz/issues/3017

Uses AdobeBlank2.ttf from:

  https://github.com/adobe-fonts/adobe-blank-2

instead of a dummy empty font so that everything maps to GID 1 and
control code points are kept instead of being dropped because there is
not space glyph (otherwise we’d need to identify control code points
somehow when generating the expectations).