kunit: tool: fix minor typing issue with None status

The code to handle aggregating statuses didn't check that the status
actually got set to some non-None value.
Default the value to SUCCESS instead of adding a bunch of `is None`
checks.

This sorta follows the precedent in commit 3fc48259

 ("kunit: Don't
fail test suites if one of them is empty").

Also slightly simplify the code and add type annotations.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>