The normal reachability manager would never get configured during
testing because it waited for AppReadiness. However, it still created
an OWSURLSession, and if between tests the previous reachability
manager wasn't destroyed fast enough, trying to set up a new one would
create a second URL session with the same background identifier, which
isn't allowed.
Fix this by providing a MockSSKReachabilityManager, which is never
reachable by default, and while here update the background identifier
to be something descriptive.
42c18bcc0b