ztest: add a weak implementation of test_main()

Introduce a weak implementation of test_main() which calls:
* ztest_run_registered_test_suites(NULL);
* ztest_verify_all_registered_test_suites_ran();

This will attempt to run all registered test suites and verify that
they each ran.

Signed-off-by: Yuval Peress <peress@chromium.org>