Make capsicum test cases fine-grained

Add a wrapping script to use ATF to run tests written with Googletest
one by one. This helps locating and tracking the failing case in CI easier.

This is a temporarily solution while Googletest support in Kyua is developing.
We will revert this once Kyua+Googletest integration is ready.

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D25896