xgettext tests: Rename xgettext-c-21 to xgettext-c-escape-3.
* gettext-tools/tests/xgettext-c-escape-3: Renamed from
gettext-tools/tests/xgettext-c-21.
* gettext-tools/tests/xg-c-escape-3.c: Renamed from
gettext-tools/tests/xg-c-21.c.
* Makefile.am (TESTS, EXTRA_DIST): Update.
