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.

src/testdir/runtest.vim

@@ -333,6 +333,7 @@ let s:flaky_tests = [
       \ 'Test_raw_one_time_callback()',
       \ 'Test_reltime()',
       \ 'Test_server_crash()',
+      \ 'Test_term_mouse_double_click_to_create_tab',
       \ 'Test_terminal_ansicolors_default()',
       \ 'Test_terminal_ansicolors_func()',
       \ 'Test_terminal_ansicolors_global()',

src/testdir/shared.vim

@@ -272,7 +272,7 @@ 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/testdir/test_substitute.vim

@@ -504,7 +504,7 @@ 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&

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1951,
 /**/
     1950,
 /**/