Macro tests only run in CI for macOS at the moment

Author: DougGregor

test/Index/index_macros.swift

@@ -1,6 +1,6 @@
 // RUN: %empty-directory(%t)
-//
 // RUN: %target-swift-ide-test -print-indexed-symbols -source-filename %s -enable-experimental-feature Macros | %FileCheck %s
+// REQUIRES: OS=macosx
 
 
 macro myLine: Int = _SwiftSyntaxMacros.LineMacro

test/Serialization/macros.swift

@@ -2,6 +2,7 @@
 // RUN: %empty-directory(%t-scratch)
 // RUN: %target-swift-frontend -emit-module -o %t/def_macros.swiftmodule %S/Inputs/def_macros.swift -module-name def_macros -enable-experimental-feature Macros
 // RUN: %target-swift-frontend -typecheck -I%t -verify %s -verify-ignore-unknown  -enable-experimental-feature Macros
+// REQUIRES: OS=macosx
 
 import def_macros