[test] Add availability guard on Character behavior test

Author: lorentey

test/stdlib/StringBridge.swift

@@ -77,6 +77,8 @@ StringBridgeTests.test("Tagged NSString") {
 
 func returnOne<T>(_ t: T) -> Int { return 1 }
 StringBridgeTests.test("Character from NSString") {
+  guard #available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *) else { return }
+
   // NOTE: Using hard-coded literals to directly construct NSStrings
   let ns1 = "A" as NSString
   let ns2 = "A\u{301}" as NSString