patch 9.1.0820: tests: Mac OS tests are too flaky

Problem:  tests: Mac OS tests are too flaky
Solution: Increase max test timeout to 25 minutes,
          allow up to 10 retries on Mac OS runners,
          refactor runtest.vim (Milly).

closes: #15940

Co-authored-by: K.Takata <kentkt@csc.jp>
Signed-off-by: Milly <milly.ca@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_channel.vim b/src/testdir/test_channel.vim
index 78c834d..2d40eff 100644
--- a/src/testdir/test_channel.vim
+++ b/src/testdir/test_channel.vim
@@ -2762,6 +2762,8 @@
 endfunc
 
 func Test_channel_lsp_mode()
+  " The channel lsp mode test is flaky and gives the same error.
+  let g:giveup_same_error = 0
   call RunServer('test_channel_lsp.py', 'LspTests', [])
 endfunc