DarwinCompatibilityTests: Set the deployment target to 10.15.7
DarwinCompatibilityTests: Set the deployment target to 10.15.7

- This allows use of new FileHandle methods that were added in 10.15.4

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