[test] xfail several stdlib tests failing in CI

10:21:57 Failing Tests (4):
10:21:57     Swift(macosx-x86_64) :: stdlib/SwiftNativeNSBase.swift
10:21:57     Swift(macosx-x86_64) :: stdlib/TestNSString.swift
10:21:57     Swift(macosx-x86_64) :: stdlib/TestJSONEncoder.swift

rdar://55727144

Author: nkcsgexi

Darwin/Foundation-swiftoverlay-Tests/TestJSONEncoder.swift

@@ -9,7 +9,7 @@
 // RUN: %target-run-simple-swift
 // REQUIRES: executable_test
 // REQUIRES: objc_interop
-
+// REQUIRES: rdar55727144
 import Swift
 import Foundation
 

Darwin/Foundation-swiftoverlay-Tests/TestNSString.swift

@@ -16,6 +16,7 @@
 // REQUIRES: executable_test
 // REQUIRES: asan_runtime
 // REQUIRES: objc_interop
+// REQUIRES: rdar55727144
 
 import Foundation
 import FoundationBridgeObjC