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/version.c

@@ -699,6 +699,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1877,
 /**/
     1876,
 /**/