libtextstyle: Add test for ostream_printf.
* libtextstyle/gnulib-local/tests/test-memory-ostream.c: New file.
* libtextstyle/gnulib-local/modules/memory-ostream-tests: New file.
* libtextstyle/Makefile.am (EXTRA_DIST): Add them.
* libtextstyle/autogen.sh (GNULIB_MODULES): Add 'memory-ostream-tests'.
