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.
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index 1df7fd9..db6dcd7 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -333,6 +333,7 @@
       \ '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()',