tests: Measure the coverage of our test suite

This is disabled by default, and needs to be enabled by passing the
--enable-gcov argument to the configure script.

Output looks like this:

  Test coverage for libcangjie2
  =============================

  Details
  -------

  cangjie.c:            82 / 146 ( 56%)
  cangjiechar.c:        11 /  11 (100%)
  cangjiecharlist.c:    33 /  33 (100%)

  Total
  -----

  Source lines:       190
  Covered statements: 126
  Total coverage:      66%

Yes, our coverage is pretty bad, but at least now we know what to
improve! :D

Fixes #22