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.

(cherry picked from commit e5b431fc0c20771a2a18cb5169d75cf337d0f1dd)