patch 8.2.0414: channel connect_waittime() test is flaky

Problem:    Channel connect_waittime() test is flaky.
Solution:   Set the test_is_flaky flag.  Use test_is_flaky for more tests.
diff --git a/src/testdir/test_terminal.vim b/src/testdir/test_terminal.vim
index e99ab7f..c922bea 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -17,6 +17,7 @@
   else
     let buf = term_start(&shell, a:options)
   endif
+  let g:test_is_flaky = 1
 
   let termlist = term_list()
   call assert_equal(1, len(termlist))