[test] test/StringIndex: Add some tests exercising replaceSubrange
[test] test/StringIndex: Add some tests exercising replaceSubrange

The exhaustive substring.replaceSubrange test probably takes too long
to include in regular testing, but let’s enable it for now: it has
caught a bunch of problems already and it will probably catch more
before this lands.

ahoppen/swift,apple/swift,apple/swift,atrick/swift,ahoppen/swift,benlangmuir/swift,benlangmuir/swift,roambotics/swift,gregomni/swift,apple/swift,roambotics/swift,glessard/swift,glessard/swift,apple/swift,gregomni/swift,atrick/swift,glessard/swift,gregomni/swift,glessard/swift,JGiola/swift,roambotics/swift,JGiola/swift,ahoppen/swift,apple/swift,gregomni/swift,ahoppen/swift,atrick/swift,JGiola/swift,glessard/swift,benlangmuir/swift,glessard/swift,ahoppen/swift,benlangmuir/swift,benlangmuir/swift,atrick/swift,apple/swift,gregomni/swift,ahoppen/swift,JGiola/swift,roambotics/swift,atrick/swift,roambotics/swift,gregomni/swift,JGiola/swift,roambotics/swift,benlangmuir/swift,JGiola/swift,atrick/swift