kunit: tool: fix pre-existing python type annotation errors

The code uses annotations, but they aren't accurate.
Note that type checking in python is a separate process, running
`kunit.py run` will not check and complain about invalid types at
runtime.

Fix pre-existing issues found by running a type checker
$ mypy *.py

All but one of these were returning `None` without denoting this
properly (via `Optional[Type]`).

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