xgettext tests: Rename xgettext-c-24 to xgettext-c-comment-6.
* gettext-tools/tests/xgettext-c-comment-6: Renamed from
gettext-tools/tests/xgettext-c-24.
* gettext-tools/tests/xg-c-comment-6.c: Renamed from
gettext-tools/tests/xg-c-24.c.
* Makefile.am (TESTS, EXTRA_DIST): Update.
