test: improve Python3 compatibility for `swift_build_sdk_interfaces`
test: improve Python3 compatibility for `swift_build_sdk_interfaces`

Return a "unicode" "string" (UTF-8 decoded data) rather than a
bytes-like object as the rest of the script expects the normal unicode
string.

benlangmuir/swift,JGiola/swift,apple/swift,jckarter/swift,JGiola/swift,airspeedswift/swift,CodaFi/swift,gregomni/swift,jmgc/swift,tkremenek/swift,nathawes/swift,apple/swift,xwu/swift,atrick/swift,nathawes/swift,allevato/swift,benlangmuir/swift,ahoppen/swift,jckarter/swift,parkera/swift,atrick/swift,parkera/swift,xwu/swift,CodaFi/swift,jmgc/swift,xwu/swift,jckarter/swift,benlangmuir/swift,JGiola/swift,hooman/swift,allevato/swift,jmgc/swift,nathawes/swift,ahoppen/swift,ahoppen/swift,parkera/swift,airspeedswift/swift,glessard/swift,allevato/swift,apple/swift,jmgc/swift,roambotics/swift,roambotics/swift,airspeedswift/swift,rudkx/swift,apple/swift,apple/swift,gregomni/swift,jmgc/swift,JGiola/swift,hooman/swift,jmgc/swift,xwu/swift,atrick/swift,atrick/swift,nathawes/swift,roambotics/swift,tkremenek/swift,JGiola/swift,glessard/swift,xwu/swift,glessard/swift,glessard/swift,jckarter/swift,nathawes/swift,atrick/swift,gregomni/swift,allevato/swift,parkera/swift,CodaFi/swift,allevato/swift,CodaFi/swift,tkremenek/swift,hooman/swift,rudkx/swift,ahoppen/swift,roambotics/swift,parkera/swift,roambotics/swift,gregomni/swift,apple/swift,jckarter/swift,airspeedswift/swift,tkremenek/swift,glessard/swift,CodaFi/swift,jmgc/swift,hooman/swift,tkremenek/swift,rudkx/swift,nathawes/swift,allevato/swift,gregomni/swift,rudkx/swift,airspeedswift/swift,roambotics/swift,jckarter/swift,ahoppen/swift,gregomni/swift,xwu/swift,airspeedswift/swift,hooman/swift,xwu/swift,CodaFi/swift,rudkx/swift,glessard/swift,parkera/swift,benlangmuir/swift,tkremenek/swift,jckarter/swift,parkera/swift,atrick/swift,benlangmuir/swift,hooman/swift,rudkx/swift,parkera/swift,tkremenek/swift,hooman/swift,JGiola/swift,nathawes/swift,allevato/swift,airspeedswift/swift,ahoppen/swift,CodaFi/swift,benlangmuir/swift