kunit: tool: fix pre-existing `mypy --strict` errors and update run_checks.py

Basically, get this command to be happy and make run_checks.py happy
 $ mypy --strict --exclude '_test.py$' --exclude qemu_configs/ ./tools/testing/kunit/

Primarily the changes are
* add `-> None` return type annotations
* add all the missing argument type annotations

Previously, we had false positives from mypy in `main()`, see commit
09641f7c ("kunit: tool: surface and address more typing issues").
But after commit 2dc9d6ca ("kunit: kunit.py extract handlers")
refactored things, the variable name reuse mypy hated is gone.

Note: mypy complains we don't annotate the types the unused args in our
signal handler. That's silly.
But to make it happy, I've copy-pasted an appropriate annotation from
https://github.com/python/typing/discussions/1042#discussioncomment-2013595

.

Reported-by: Johannes Berg <johannes.berg@intel.com>
Link: https://lore.kernel.org/linux-kselftest/9a172b50457f4074af41fe1dc8e55dcaf4795d7e.camel@sipsolutions.net/


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