Add testfx to VMR's source mappings (#45429)

Author: nohwnd

src/VirtualMonoRepo/source-mappings.json

@@ -158,6 +158,10 @@
             "name": "test-templates",
             "defaultRemote": "https://github.com/dotnet/test-templates"
         },
+        {
+            "name": "testfx",
+            "defaultRemote": "https://github.com/microsoft/testfx"
+        },
         {
             "name": "vstest",
             "defaultRemote": "https://github.com/microsoft/vstest",