Change syncback_service text fixture scope.
Change syncback_service text fixture scope.

I still really don't understand how exactly gevent and pytest interact, but
this seems to resolve the issue in which later tests using the fixture fail
when the whole suite is run.

gale320/sync-engine,nylas/sync-engine,wakermahmud/sync-engine,wakermahmud/sync-engine,Eagles2F/sync-engine,gale320/sync-engine,PriviPK/privipk-sync-engine,EthanBlackburn/sync-engine,ErinCall/sync-engine,wakermahmud/sync-engine,ErinCall/sync-engine,EthanBlackburn/sync-engine,Eagles2F/sync-engine,Eagles2F/sync-engine,jobscore/sync-engine,PriviPK/privipk-sync-engine,closeio/nylas,ErinCall/sync-engine,EthanBlackburn/sync-engine,PriviPK/privipk-sync-engine,nylas/sync-engine,closeio/nylas,EthanBlackburn/sync-engine,gale320/sync-engine,Eagles2F/sync-engine,closeio/nylas,ErinCall/sync-engine,PriviPK/privipk-sync-engine,jobscore/sync-engine,jobscore/sync-engine,jobscore/sync-engine,nylas/sync-engine,wakermahmud/sync-engine,Eagles2F/sync-engine,ErinCall/sync-engine,wakermahmud/sync-engine,gale320/sync-engine,closeio/nylas,EthanBlackburn/sync-engine,PriviPK/privipk-sync-engine,nylas/sync-engine,gale320/sync-engine