Re-enable emoji font tests

This re-enables the emoji font tests, but doesn't run the tests if
SMALLER_FONT_FOOTPRINT is set.

Additionally, we test that there's one and only one emoji font in the
fallback chain.

Bug: 26187231
Bug: 27785690
Change-Id: I2e6d467e3f54ee3d41737170d7c2492a0f6b30f0