make: add a target for building pyoxidizer tests on macOS
The resources seem to be embedded inside the binary, but for some reasons they
aren't read there. And since they are embedded, they aren't staged by the build
in the `lib` directory like on Windows. So copy them from the repo. We can
figure out what's going wrong later.
#require docutils gettext
$ $TESTDIR/check-gendoc zh_TW
checking for parse errors