tests: add a basic test for argc == 0

The kernel should reject such exec()s now, early on. Instead of adding
the needed boilerplate to write a test in C, just add an -n argument for
"(n)ull argv" to the execve helper and exec this other helper that just
exits silently with argv count.

Reviewed by: emaste, kib, markj (all previous version)
Differential Revision: https://reviews.freebsd.org/D34045