kunit: tool: stop using bare asserts in unit test

Use self.assertEqual/assertNotEqual() instead.
Besides being more appropriate in a unit test, it'll also give a better
error message by show the unexpected values.

Also
* Delete redundant check of exception types. self.assertRaises does this.
* s/kall/call. There's no reason to name it this way.
  * This is probably a misunderstanding from the docs which uses it
  since `mock.call` is in scope as `call`.

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