selftests: Extract logic for multiple test runs

This moves the logic for running multiple tests into a single "run_many"
function of runner.sh. Both "run_tests" and "emit_tests" are modified to
use it. Summary handling is now controlled by the "per_test_logging"
shell flag.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>