Add missing shebang

Plain test programs are not preprocessed by the build system (as opposed to
ATF test cases, which automatically gain a shebang pointing at atf-sh), so
we must take care of providing the shebang ourselves.

I'm not sure why this was not causing problems with Kyua 0.11, but the
upcoming 0.12 release chokes on this particular issue.

MFC after: 1 week