Enable address sanitizer by default for Mac unit tests

GCDWebServer.xcodeproj/xcshareddata/xcschemes/GCDWebServers (Mac).xcscheme

@@ -26,7 +26,8 @@
       buildConfiguration = "Debug"
       selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
       selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
-      shouldUseLaunchSchemeArgsEnv = "YES">
+      shouldUseLaunchSchemeArgsEnv = "YES"
+      enableAddressSanitizer = "YES">
       <Testables>
          <TestableReference
             skipped = "NO">