Tests: Fix tests with incorrect REQUIRES=macosx lines.

Author: tshortli

test/ClangImporter/availability_platform_categories.swift

@@ -1,6 +1,6 @@
 // RUN: %target-swift-frontend -typecheck %s -import-objc-header %S/Inputs/availability_platform_categories.h
 
-// REQUIRES: OS=macos
+// REQUIRES: OS=macosx
 
 // Test when a function is associated to an Objective-C category with the
 // wrong unavailability. rdar://problem/53956555

test/SILOptimizer/moveonly_addresschecker_tsan.swift

@@ -1,6 +1,6 @@
 // RUN: %target-swift-emit-sil -sanitize=thread -sil-verify-all -verify -enable-experimental-feature NoImplicitCopy -enable-experimental-feature MoveOnlyClasses %s -Xllvm -sil-print-final-ossa-module | %FileCheck %s
 // RUN: %target-swift-emit-sil -sanitize=thread -O -sil-verify-all -verify -enable-experimental-feature NoImplicitCopy -enable-experimental-feature MoveOnlyClasses %s
-// REQUIRES: OS=macOS
+// REQUIRES: OS=macosx
 
 // This file contains tests that used to crash due to verifier errors. It must
 // be separate from moveonly_addresschecker_diagnostics since when we fail on