vim-patch:9.0.1877: missing test for patch 9.0.1873

Problem:  missing test for patch 9.0.1873
Solution: add a test trying to exchange windows

Add a test, making sure that switching windows is not allowed when
textlock is active, e.g. when running `:s/<pat>/\=func()/`

18d2709aa1

Co-authored-by: Christian Brabandt <cb@256bit.org>

src/nvim/window.c

@@ -1884,6 +1884,10 @@ static void win_exchange(int Prenum)
     beep_flush();
     return;
   }
+  if (text_or_buf_locked()) {
+    beep_flush();
+    return;
+  }
 
   frame_T *frp;