kunit: tool: make --json handling a bit clearer

Currently kunit_json.get_json_result() will output the JSON-ified test
output to json_path, but iff it's not "stdout".

Instead, move the responsibility entirely over to the one caller.

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