Fix the Order of Arguments to a Test Assertion

[mdimjasevic]

This is a rather small and insignificant change, hence I haven't added a change log.

The one-line change just puts the expected and actual value arguments to (@?=) in the correct order in a federation test.

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• If end-points have been added or changed: the endpoint / config-flag checklist (see Wire-employee only backend wiki page) has been followed.
• If a schema migration has been added, I ran make git-add-cassandra-schema to update the cassandra schema documentation.

[fisx]

This is hillarious: HUnit has both @?= and @=?, just in case anybody's not confused enough by the one option. :')

[mdimjasevic]

This is hillarious: HUnit has both @?= and @=?, just in case anybody's not confused enough by the one option. :')

And their arguments are swapped!

Because of this, I always end up having to look up the documentation to see which argument is which. I spotted this error only because I was copy-pasting the affected test, had an unfinished implementation, and noticed my test failing, but with an unexpected expected value.