kunit: tool: make TestCounts a dataclass

Since we're using Python 3.7+, we can use dataclasses to tersen the
code.

It also lets us create pre-populated TestCounts() objects and compare
them in our unit test. (Before, you could only create empty ones).

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