xccdf mapper should properly implement 'check' subdescription

[Amndeep7]

for the compliance as code sample, the attribute we look for for the check subdescription is not there so it falls back to the tag.

whereas the ref/reference stuff is not overlapped, the check subdescription and check tag do have overlap where if the subdescription is available it hides the tag. for now, we have stringified the tag so it looks good in heimdall, but we should go and make the subdescription find things properly and behave well with the original data being an array for example.

Originally posted by @Amndeep7 in #4255 (comment)