Add many more emoji tests

This creates an exact map of the emoji font and adds the following
tests:
* Test that all 'emoji style' variation sequences exist in the color
  emoji font.
* Test that the emoji font contains the exact same character set that
  we expect.
* Test that all unsupported flags map to the same flag tofu glyph.
* Test that the reverse of ZWJ sequences exist in the font and map to
  the same glyph.
* Test that legacy PUA Android emoji are supported.
* Test that all emoji sequences that should map to the same glyph do
  so, but no other sequences map to the same glyph.

Bug: 26187231
Change-Id: I64ef17862df2d91879bf7904055c77c24abb7d88