TestFoundation: replace `CFStringCreateExternalRepresentation`
TestFoundation: replace `CFStringCreateExternalRepresentation`

Use `NSString(dataUsingStringEncoding:allowLossyConversion:)` instead to
construct the external representation.  This avoids the use of
CoreFoundation APIs.

apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,e78l/swift-corelibs-foundation,e78l/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,e78l/swift-corelibs-foundation,e78l/swift-corelibs-foundation,e78l/swift-corelibs-foundation