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()/`

Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_substitute.vim

@@ -1439,4 +1439,18 @@ func Test_z_substitute_expr_leak()
   delfunc SubExpr
 endfunc
 
+func Test_substitute_expr_switch_win()
+  func R()
+    wincmd x
+    return 'XXXX'
+  endfunc
+  new Xfoobar
+  let bufnr = bufnr('%')
+  put ="abcdef"
+  silent! s/\%')/\=R()
+  call assert_fails(':%s/./\=R()/g', 'E565')
+  delfunc R
+  exe bufnr .. "bw!"
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab