Fix build for Linux

In order for the test to build/run, I needed to add an Execveat subclass of
Execve, then use TEST_F(..) for the test in order to leverage exec_prog_.
This wasn't caught previously, because FreeBSD lacks an execveat(2) call,
unlike Linux.

Reported by: David Drysdale <drysdale@google.com>
Pull Request: https://github.com/google/capsicum-test/pull/41 (WIP)