vim-patch:8.1.2148: no test for right click extending Visual area

Problem:    No test for right click extending Visual area.
Solution:   Add a test. (Dominique Pelle, closes vim/vim#5018)

6aa7523b96

Co-authored-by: Bram Moolenaar <Bram@vim.org>

test/old/testdir/setup.vim

@@ -12,6 +12,7 @@ if exists('s:did_load')
   set laststatus=1
   set listchars=eol:$
   set joinspaces
+  set mousemodel=extend
   set nohidden nosmarttab noautoindent noautoread noruler noshowcmd
   set nohlsearch noincsearch
   set nrformats=bin,octal,hex