Add feature specs for combined trial on premium

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/169003



Merged-by: Aaron Huntsman <ahuntsman@gitlab.com>
Approved-by: Olaoluwa Oluro <olaoluro@gitlab.com>
Approved-by: Aaron Huntsman <ahuntsman@gitlab.com>
Reviewed-by: Olaoluwa Oluro <olaoluro@gitlab.com>
Co-authored-by: david hamp-gonsalves <dhamp-gonsalves@gitlab.com>