While implementing https://jira.mongodb.org/browse/RUBY-1258, we noticed that there appears to be no test in the auth spec tests for an invalid mechanism. We have previously identified that the Ruby driver would not report an invalid auth mechanism until an operation was attempted (https://jira.mongodb.org/browse/RUBY-1745). Since the auth spec requires the drivers to report auth (credential)-related errors as early as possible, it seems appropriate to also include a test for an invalid auth mechanism in the spec tests. (The existing spec tests check for invalid and missing auth sources for valid mechanisms, but not for invalid mechanisms).