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/ex_cmds.c

@@ -3888,6 +3888,10 @@ static int do_sub(exarg_T *eap, const proftime_T timeout, const long cmdpreview_
           nmatch = curbuf->b_ml.ml_line_count - sub_firstlnum + 1;
           current_match.end.lnum = sub_firstlnum + (linenr_T)nmatch;
           skip_match = true;
+          // safety check
+          if (nmatch < 0) {
+            goto skip;
+          }
         }
 
         // Save the line numbers for the preview buffer