MFC r285118:
r285118 (by jmmv):

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.