vim-patch:8.1.1951: mouse double click test is a bit flaky

Problem:    Mouse double click test is a bit flaky.
Solution:   Add to list of flaky tests.  Update a couple of comments.
f1699968ba

src/nvim/testdir/shared.vim

@@ -279,7 +279,7 @@ func GetVimCommand(...)
   return cmd
 endfunc
 
-" Get the command to run Vim, with --clean.
+" Get the command to run Vim, with --clean instead of "-u NONE".
 func GetVimCommandClean()
   let cmd = GetVimCommand()
   let cmd = substitute(cmd, '-u NONE', '--clean', '')