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/runtest.vim

@@ -297,6 +297,7 @@ let s:flaky_tests = [
       \ 'Test_repeat_three()',
       \ 'Test_state()',
       \ 'Test_stop_all_in_callback()',
+      \ 'Test_term_mouse_double_click_to_create_tab',
       \ 'Test_term_mouse_multiple_clicks_to_visually_select()',
       \ 'Test_terminal_composing_unicode()',
       \ 'Test_terminal_redir_file()',

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', '')

src/nvim/testdir/test_substitute.vim

@@ -241,7 +241,7 @@ func Test_sub_cmd_3()
   call Run_SubCmd_Tests(tests)
 endfunc
 
-" Test for submatch() on :substitue.
+" Test for submatch() on :substitute.
 func Test_sub_cmd_4()
   set magic&
   set cpo&