make included_with_xcode? more testable

libraries/xcode.rb

@@ -86,10 +86,13 @@ module MacOS
         shell_out!(XCVersion.list_simulators).stdout
       end
 
+      def show_sdks
+        shell_out!('/usr/bin/xcodebuild -showsdks').stdout
+      end
+
       def included_with_xcode?
         version_matcher    = /\d{1,2}\.\d{0,2}\.?\d{0,3}/
-        sdks               = shell_out!('/usr/bin/xcodebuild -showsdks').stdout
-        included_simulator = sdks.match(/Simulator - iOS (?<version>#{version_matcher})/)
+        included_simulator = show_sdks.match(/Simulator - iOS (?<version>#{version_matcher})/)
         @version.split.last.to_i == included_simulator[:version].to_i
       end
     end