doc: Mention --color=test in the description of the --color option.
Reported by Nathaniel M. Beaver <nathanielmbeaver@gmail.com>
in <https://savannah.gnu.org/bugs/?55573>.
* gettext-tools/doc/gettext.texi (Colorizing): Mention that the --color and
--style options are applicable to many programs.
(The --color option): Mention the 'test' value.
