Fix reflection use in integration tests (#77669)

In
https://devdiv.visualstudio.com/DevDiv/_git/VSEditor/pullrequest/618447
the field name was changed, so tests will break in future. We just got
broken by this in Razor because we test against VS main
(dotnet/razor#11640) so figured I'd prevent this
breaking Roslyn too, at whatever point in the future the VS image is
updated.

Author: davidwengier

src/VisualStudio/IntegrationTest/New.IntegrationTests/InProcess/FindReferencesWindowInProcess.cs

@@ -82,7 +82,7 @@ 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") ?? toolWindowType.GetField("Control");
         var toolWindowControl = toolWindowControlField.GetValue(toolWindow);
 
         // Dig further to get the results table (as opposed to the toolbar).