[test] Replace some more 9999 availabilities with SwiftStdlib 5.6

Author: lorentey

test/stdlib/CodableTests.swift

@@ -480,7 +480,7 @@ class TestCodable : TestCodableSuper {
         }
     }
 
-    @available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
+    @available(SwiftStdlib 5.6, *)
     func test_Dictionary_JSON() {
         enum X: String, Codable { case a, b }
         enum Y: String, Codable, CodingKeyRepresentable { case a, b }
@@ -1054,7 +1054,7 @@ if #available(macOS 10.12, iOS 10.0, watchOS 3.0, tvOS 10.0, *) {
     tests["test_URLComponents_Plist"] = TestCodable.test_URLComponents_Plist
 }
 
-if #available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *) {
+if #available(SwiftStdlib 5.6, *) {
     tests["test_Dictionary_JSON"] = TestCodable.test_Dictionary_JSON
 }
 

test/stdlib/RawRepresentable-tricky-hashing.swift

@@ -107,7 +107,7 @@ suite.test("hashValue forwarding") {
   //
   // See https://github.com/apple/swift/pull/39155
 
-  if #available(macOS 9999, iOS 9999, tvOS 9999, watchOS 9999, *) {
+  if #available(SwiftStdlib 5.6, *) {
     let r = CustomRawRepresentable2(rawValue: Bogus())!
     expectEqual(r.hashValue, 23.hashValue)
   }

validation-test/stdlib/Unicode.swift.gyb

@@ -212,7 +212,7 @@ UnicodeScalarTests.test("init") {
   expectEqual(nil, UnicodeScalar(UInt32(0xD800)))
   expectEqual("j", Unicode.Scalar(Int(0x6A)))
 
-  if #available(macOS 9999, iOS 9999, tvOS 9999, watchOS 9999, *) {
+  if #available(SwiftStdlib 5.6, *) {
     expectEqual(nil, Unicode.Scalar(-0x6A))
   }
 }