kunit: tool: drop mostly unused KunitResult.result field

This field is only used to pass along the parsed Test object from
parse_tests().
Everywhere else the `result` field is ignored.

Instead make parse_tests() explicitly return a KunitResult and Test so
we can retire the `result` field.

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