[devops] Enable legacy xamarin tests by default. (#16115)

Now that it's possible to disable legacy xamarin tests, we must also
enable
them after using 'skip-all-tests' if we want to run legacy tests.

Otherwise we end up only executing .NET tests:

f592de721d (commitcomment-84832124)

tools/devops/automation/templates/main-stage.yml

@@ -198,7 +198,7 @@ stages:
     displayName: 'Simulator tests'
     testPool: '' # use the default
     useXamarinStorage: false
-    testsLabels: '--label=skip-all-tests,run-ios-64-tests,run-ios-simulator-tests,run-tvos-tests,run-watchos-tests,run-mac-tests,run-maccatalyst-tests,run-dotnet-tests,run-system-permission-tests'
+    testsLabels: '--label=skip-all-tests,run-ios-64-tests,run-ios-simulator-tests,run-tvos-tests,run-watchos-tests,run-mac-tests,run-maccatalyst-tests,run-dotnet-tests,run-system-permission-tests,run-legacy-xamarin-tests'
     statusContext: 'VSTS: simulator tests'
     makeTarget: 'jenkins'
     vsdropsPrefix: ${{ variables.vsdropsPrefix }}