Distribute the source code of csharpexec-test.exe.
Reported by Bastien Roucariès <rouca@debian.org>
via Santiago Vila <sanvila@debian.org>
in <https://lists.gnu.org/archive/html/bug-gettext/2023-02/msg00016.html>.
* gettext-tools/m4/csharpexec-test.cs: New file, extracted from pre-2018
versions of gnulib/m4/csharpcomp.m4.
* gettext-tools/m4/Makefile.am (EXTRA_DIST): Add it.
