kunit: tool: make --json do nothing if --raw_ouput is set

When --raw_output is set (to any value), we don't actually parse the
test results. So asking to print the test results as json doesn't make
sense.

We internally create a fake test with one passing subtest, so --json
would actually print out something misleading.

This patch:
* Rewords the flag descriptions so hopefully this is more obvious.
* Also updates --raw_output's description to note the default behavior
  is to print out only "KUnit" results (actually any KTAP results)
* also renames and refactors some related logic for clarity (e.g.
  test_result => test, it's a kunit_parser.Test object).

Notably, this patch does not make it an error to specify --json and
--raw_output together. This is an edge case, but I know of at least one
wrapper around kunit.py that always sets --json. You'd never be able to
use --raw_output with that wrapper.

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