unigbrk: Fix uc_graphemeclusterbreak_property(), add comprehensive test.

unigbrk: Fix uc_graphemeclusterbreak_property(), add comprehensive test.

gbrkprop: Fix implementation of uc_graphemeclusterbreak_property.
* lib/unigbrk/gbrkprop.h: Regenerate with gen-uni-tables.c.  I had
modified how this file was generated before I initially submitted
the module, but failed to regenerate it.  This meant that several
of the level2 entries were wrong.
* lib/unigbrk/uc-gbrk-prop.h (uc_graphemeclusterbreak_property):
Remove the division-by-2 that is folded into the table now that
gbrkprop.h has been regenerated properly.  Now -1 entries are
handled correctly.

New module 'unigbrk/uc-gbrk-prop-tests'.
* modules/unigbrk/uc-gbrk-prop-tests: New file.
* lib/gen-uni-tables.c: Generate tests/test-uc-gbrk-prop.h.
* tests/unigbrk/test-uc-gbrk-prop.c: New file.
* tests/unigbrk/test-uc-gbrk-prop.h: New file.