test: add a helper to join and normalise paths (NFC)
test: add a helper to join and normalise paths (NFC)

This ensures that all paths that are generated in lit are normalised
after being formed.  This is in preparation for supporting execution of
the tests on Windows.

rudkx/swift,nathawes/swift,apple/swift,gregomni/swift,shahmishal/swift,practicalswift/swift,roambotics/swift,amraboelela/swift,JGiola/swift,danielmartin/swift,gregomni/swift,huonw/swift,amraboelela/swift,austinzheng/swift,practicalswift/swift,jckarter/swift,jopamer/swift,benlangmuir/swift,sschiau/swift,ahoppen/swift,sschiau/swift,airspeedswift/swift,shajrawi/swift,devincoughlin/swift,danielmartin/swift,roambotics/swift,brentdax/swift,CodaFi/swift,xedin/swift,gribozavr/swift,CodaFi/swift,apple/swift,alblue/swift,glessard/swift,rudkx/swift,karwa/swift,xwu/swift,nathawes/swift,hooman/swift,shahmishal/swift,gribozavr/swift,tkremenek/swift,shahmishal/swift,benlangmuir/swift,tkremenek/swift,practicalswift/swift,airspeedswift/swift,austinzheng/swift,CodaFi/swift,devincoughlin/swift,JGiola/swift,lorentey/swift,atrick/swift,karwa/swift,danielmartin/swift,sschiau/swift,nathawes/swift,austinzheng/swift,hooman/swift,xwu/swift,atrick/swift,ahoppen/swift,roambotics/swift,shahmishal/swift,gregomni/swift,practicalswift/swift,aschwaighofer/swift,jmgc/swift,sschiau/swift,xwu/swift,parkera/swift,atrick/swift,hooman/swift,CodaFi/swift,brentdax/swift,ahoppen/swift,alblue/swift,allevato/swift,amraboelela/swift,stephentyrone/swift,atrick/swift,austinzheng/swift,practicalswift/swift,xedin/swift,danielmartin/swift,xedin/swift,parkera/swift,airspeedswift/swift,sschiau/swift,devincoughlin/swift,natecook1000/swift,hooman/swift,jopamer/swift,parkera/swift,roambotics/swift,gribozavr/swift,devincoughlin/swift,apple/swift,hooman/swift,ahoppen/swift,parkera/swift,xwu/swift,jopamer/swift,stephentyrone/swift,jmgc/swift,alblue/swift,harlanhaskins/swift,apple/swift,devincoughlin/swift,nathawes/swift,JGiola/swift,harlanhaskins/swift,natecook1000/swift,tkremenek/swift,glessard/swift,glessard/swift,gribozavr/swift,atrick/swift,ahoppen/swift,lorentey/swift,lorentey/swift,roambotics/swift,roambotics/swift,shahmishal/swift,devincoughlin/swift,stephentyrone/swift,jckarter/swift,natecook1000/swift,amraboelela/swift,stephentyrone/swift,glessard/swift,shajrawi/swift,natecook1000/swift,austinzheng/swift,tkremenek/swift,gregomni/swift,gregomni/swift,alblue/swift,austinzheng/swift,parkera/swift,aschwaighofer/swift,parkera/swift,airspeedswift/swift,stephentyrone/swift,austinzheng/swift,jckarter/swift,danielmartin/swift,shajrawi/swift,airspeedswift/swift,huonw/swift,apple/swift,allevato/swift,alblue/swift,JGiola/swift,rudkx/swift,tkremenek/swift,sschiau/swift,CodaFi/swift,practicalswift/swift,brentdax/swift,shajrawi/swift,jopamer/swift,tkremenek/swift,huonw/swift,CodaFi/swift,hooman/swift,apple/swift,devincoughlin/swift,shajrawi/swift,stephentyrone/swift,lorentey/swift,shajrawi/swift,xedin/swift,danielmartin/swift,karwa/swift,xedin/swift,shahmishal/swift,xedin/swift,natecook1000/swift,airspeedswift/swift,practicalswift/swift,danielmartin/swift,aschwaighofer/swift,stephentyrone/swift,atrick/swift,jckarter/swift,harlanhaskins/swift,gribozavr/swift,amraboelela/swift,jopamer/swift,gribozavr/swift,jopamer/swift,shajrawi/swift,gribozavr/swift,harlanhaskins/swift,airspeedswift/swift,harlanhaskins/swift,natecook1000/swift,xwu/swift,karwa/swift,allevato/swift,jmgc/swift,tkremenek/swift,allevato/swift,shahmishal/swift,allevato/swift,lorentey/swift,xedin/swift,karwa/swift,hooman/swift,gribozavr/swift,glessard/swift,amraboelela/swift,jckarter/swift,jckarter/swift,jmgc/swift,CodaFi/swift,jmgc/swift,lorentey/swift,allevato/swift,aschwaighofer/swift,brentdax/swift,glessard/swift,jopamer/swift,natecook1000/swift,karwa/swift,JGiola/swift,brentdax/swift,xwu/swift,huonw/swift,allevato/swift,brentdax/swift,parkera/swift,aschwaighofer/swift,benlangmuir/swift,ahoppen/swift,harlanhaskins/swift,devincoughlin/swift,JGiola/swift,huonw/swift,brentdax/swift,jckarter/swift,sschiau/swift,karwa/swift,nathawes/swift,parkera/swift,xedin/swift,benlangmuir/swift,karwa/swift,benlangmuir/swift,xwu/swift,jmgc/swift,nathawes/swift,sschiau/swift,gregomni/swift,shahmishal/swift,benlangmuir/swift,lorentey/swift,harlanhaskins/swift,nathawes/swift,amraboelela/swift,alblue/swift,rudkx/swift,shajrawi/swift,aschwaighofer/swift,alblue/swift,huonw/swift,practicalswift/swift,jmgc/swift,rudkx/swift,huonw/swift,lorentey/swift,aschwaighofer/swift,rudkx/swift