Disable async completion for 16.0 Preview 3 testing due to bugs

src/VisualStudio/IntegrationTest/TestUtilities/VisualStudioInstanceFactory.cs

@@ -324,6 +324,9 @@ private static Process StartNewVisualStudioProcess(string installationPath, int
                 // Disable roaming settings to avoid interference from the online user profile
                 Process.Start(vsRegEditExeFile, $"set \"{installationPath}\" {Settings.Default.VsRootSuffix} HKCU \"ApplicationPrivateSettings\\Microsoft\\VisualStudio\" RoamingEnabled string \"1*System.Boolean*False\"").WaitForExit();
 
+                // Disable async completion for 16.0 Preview 3 testing
+                Process.Start(vsRegEditExeFile, $"set \"{installationPath}\" {Settings.Default.VsRootSuffix} HKCU \"ApplicationPrivateSettings\\WindowManagement\\Options\" UseAsyncCompletion string \"1*System.Int32*-1\"").WaitForExit();
+
                 // Disable text editor error reporting because it pops up a dialog. We want to either fail fast in our
                 // custom handler or fail silently and continue testing.
                 Process.Start(vsRegEditExeFile, $"set \"{installationPath}\" {Settings.Default.VsRootSuffix} HKCU \"Text Editor\" \"Report Exceptions\" dword 0").WaitForExit();