Fix find all references integration tests (#11640)

https://devdiv.visualstudio.com/DevDiv/_git/VSEditor/pullrequest/618447
renamed a field

Author: davidwengier

src/Razor/test/Microsoft.VisualStudio.Razor.IntegrationTests/InProcess/FindReferencesWindowInProcess.cs

@@ -81,7 +81,7 @@ private async Task<IWpfTableControl2> GetFindReferencesWindowAsync(CancellationT
 
         // Dig through to get the Find References control.
         var toolWindowType = toolWindow.GetType();
-        var toolWindowControlField = toolWindowType.GetField("Control");
+        var toolWindowControlField = toolWindowType.GetField("_control");
         var toolWindowControl = toolWindowControlField.GetValue(toolWindow);
 
         // Dig further to get the results table (as opposed to the toolbar).