Add some unit tests.

- Type "tgis", manually pick "thus", press backspace.
  Backspace should revert the manual pick.
  Check "tgis" is the result.
- Type "tgis" followed by a period.
  Period should trigger auto-correct to "this".
  Check "this" is the result.
- Type "tgis" followed by a period, then backspace.
  Period should trigger auto-correct to "this", and backspace should
  revert the auto-correction.
  Check "tgis." is the result.

Change-Id: I7e23c8a26fbdbe23336149a05ff01bc51707422e