SR-1674: Add @testable import to TestNSString.swift

- This allows access to internal methods and properties of Foundation.

Author: spevans

TestFoundation/TestNSString.swift

@@ -9,6 +9,12 @@
 
 import CoreFoundation
 
+#if DEPLOYMENT_RUNTIME_OBJC || os(Linux) || os(Android)
+@testable import Foundation
+#else
+@testable import SwiftFoundation
+#endif
+
 #if os(macOS) || os(iOS)
 internal let kCFStringEncodingMacRoman =  CFStringBuiltInEncodings.macRoman.rawValue
 internal let kCFStringEncodingWindowsLatin1 =  CFStringBuiltInEncodings.windowsLatin1.rawValue