Improve audit event spec for group saml provider changes

Follow-up from "Add group SAML configuration changes to group audit events"

The following discussion from !73656 (merged) should be addressed:

• @tkuah started a discussion:

  I think we lost the assertion on the message for the audit event. At the least we should assert that each message contains the right attribute name. Bonus if we can assert on attribute value. (see !73656 (comment 731279322) for previous context)

  Happy to follow-up here