[xctest] Check for an error message prefix only
[xctest] Check for an error message prefix only

Checking the whole message is fragile and might break should the
formatting change ever so slightly.

Fixes: <rdar://problem/46030955>

roambotics/swift,benlangmuir/swift,allevato/swift,JGiola/swift,practicalswift/swift,tkremenek/swift,gregomni/swift,rudkx/swift,hooman/swift,atrick/swift,JGiola/swift,danielmartin/swift,benlangmuir/swift,glessard/swift,tkremenek/swift,apple/swift,sschiau/swift,sschiau/swift,lorentey/swift,jckarter/swift,austinzheng/swift,karwa/swift,JGiola/swift,gregomni/swift,nathawes/swift,sschiau/swift,gribozavr/swift,glessard/swift,aschwaighofer/swift,tkremenek/swift,xwu/swift,shahmishal/swift,shajrawi/swift,atrick/swift,airspeedswift/swift,CodaFi/swift,lorentey/swift,parkera/swift,CodaFi/swift,tkremenek/swift,gribozavr/swift,amraboelela/swift,shahmishal/swift,harlanhaskins/swift,hooman/swift,stephentyrone/swift,ahoppen/swift,jmgc/swift,benlangmuir/swift,amraboelela/swift,xedin/swift,gribozavr/swift,stephentyrone/swift,gribozavr/swift,allevato/swift,nathawes/swift,harlanhaskins/swift,xedin/swift,parkera/swift,aschwaighofer/swift,roambotics/swift,devincoughlin/swift,xwu/swift,austinzheng/swift,airspeedswift/swift,apple/swift,devincoughlin/swift,airspeedswift/swift,apple/swift,karwa/swift,xwu/swift,stephentyrone/swift,allevato/swift,jmgc/swift,practicalswift/swift,harlanhaskins/swift,lorentey/swift,jckarter/swift,stephentyrone/swift,airspeedswift/swift,devincoughlin/swift,roambotics/swift,glessard/swift,hooman/swift,rudkx/swift,ahoppen/swift,nathawes/swift,karwa/swift,xwu/swift,karwa/swift,shahmishal/swift,allevato/swift,aschwaighofer/swift,aschwaighofer/swift,shajrawi/swift,roambotics/swift,aschwaighofer/swift,amraboelela/swift,CodaFi/swift,airspeedswift/swift,atrick/swift,practicalswift/swift,allevato/swift,shahmishal/swift,rudkx/swift,devincoughlin/swift,shahmishal/swift,lorentey/swift,sschiau/swift,xedin/swift,xedin/swift,lorentey/swift,JGiola/swift,parkera/swift,shajrawi/swift,ahoppen/swift,airspeedswift/swift,ahoppen/swift,atrick/swift,glessard/swift,sschiau/swift,nathawes/swift,JGiola/swift,nathawes/swift,gribozavr/swift,nathawes/swift,benlangmuir/swift,xedin/swift,practicalswift/swift,atrick/swift,lorentey/swift,lorentey/swift,shajrawi/swift,devincoughlin/swift,xwu/swift,harlanhaskins/swift,shajrawi/swift,jckarter/swift,devincoughlin/swift,karwa/swift,rudkx/swift,CodaFi/swift,amraboelela/swift,CodaFi/swift,jmgc/swift,danielmartin/swift,harlanhaskins/swift,ahoppen/swift,hooman/swift,jmgc/swift,hooman/swift,practicalswift/swift,karwa/swift,aschwaighofer/swift,hooman/swift,devincoughlin/swift,nathawes/swift,austinzheng/swift,xedin/swift,shajrawi/swift,rudkx/swift,amraboelela/swift,xwu/swift,roambotics/swift,xwu/swift,jckarter/swift,gregomni/swift,stephentyrone/swift,gregomni/swift,roambotics/swift,danielmartin/swift,danielmartin/swift,shajrawi/swift,gregomni/swift,parkera/swift,atrick/swift,gribozavr/swift,jmgc/swift,karwa/swift,practicalswift/swift,apple/swift,CodaFi/swift,harlanhaskins/swift,apple/swift,jckarter/swift,parkera/swift,stephentyrone/swift,JGiola/swift,practicalswift/swift,rudkx/swift,devincoughlin/swift,sschiau/swift,xedin/swift,allevato/swift,parkera/swift,airspeedswift/swift,sschiau/swift,jckarter/swift,stephentyrone/swift,allevato/swift,jmgc/swift,tkremenek/swift,benlangmuir/swift,danielmartin/swift,shahmishal/swift,glessard/swift,sschiau/swift,austinzheng/swift,jckarter/swift,jmgc/swift,tkremenek/swift,tkremenek/swift,karwa/swift,hooman/swift,austinzheng/swift,amraboelela/swift,danielmartin/swift,austinzheng/swift,harlanhaskins/swift,ahoppen/swift,gribozavr/swift,benlangmuir/swift,amraboelela/swift,parkera/swift,apple/swift,parkera/swift,danielmartin/swift,glessard/swift,CodaFi/swift,xedin/swift,gregomni/swift,practicalswift/swift,lorentey/swift,austinzheng/swift,shahmishal/swift,shahmishal/swift,aschwaighofer/swift,gribozavr/swift,shajrawi/swift