xgettext: New test of concatenation of strings with escape sequences.
Reported by Morten Welinder <mwelinder@gmail.com>
in <https://lists.gnu.org/archive/html/bug-gettext/2015-12/msg00017.html>
and <https://savannah.gnu.org/bugs/?46756>.
* gettext-tools/tests/xgettext-c-23: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add it.
