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_channel.vim b/src/testdir/test_channel.vim
index a820be5..abc27d5 100644
--- a/src/testdir/test_channel.vim
+++ b/src/testdir/test_channel.vim
@@ -23,6 +23,9 @@
 " Run "testfunc" after starting the server and stop the server afterwards.
 func s:run_server(testfunc, ...)
   call RunServer('test_channel.py', a:testfunc, a:000)
+
+  " communicating with a server can be flaky
+  let g:test_is_flaky = 1
 endfunc
 
 " Return a list of open files.
@@ -415,6 +418,8 @@
 " Test that trying to connect to a non-existing port fails quickly.
 func Test_connect_waittime()
   CheckFunction reltimefloat
+  " this is timing sensitive
+  let g:test_is_flaky = 1
 
   call ch_log('Test_connect_waittime()')
   let start = reltime()