testing: provide meaningful error when pytest is not available

atf format does not provide any way of signalling any error message
back to the atf runner when listing tests. Work this around by
reporting "test_cases_list_pytest_binary_not_found" test instead.

Reviewed By: kp
Differential Revision: https://reviews.freebsd.org/D35721

(cherry picked from commit 2bfd8b5b9419b0ceb3dd0295fdf413d32969e5b2)