Add integration tests for global.json --test-runner option

Co-authored-by: Youssef1313 <[email protected]>

Author: Copilot

template_feed/Microsoft.DotNet.Common.ItemTemplates/content/GlobalJson/global.json

@@ -4,7 +4,8 @@
     "rollForward": "ROLL_FORWARD_VALUE",
     //#endif
     "version": "SDK_VERSION"
-  }//#if (TestRunner!=""),
+  }//#if (TestRunner!="")
+,
   "test": {
     "runner": "TEST_RUNNER_VALUE"
   }