kunit: tool: simplify kconfig is_subset_of() logic

Don't use an O(nm) algorithm* and make it more readable by using a dict.

*Most obviously, it does a nested for-loop over the entire other config.
A bit more subtle, it calls .entries(), which constructs a set from the
list for _every_ outer iteration.

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>