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/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c1cca40..6b3b3d0 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -389,7 +389,7 @@
           brew install diffutils
 
       - name: Test
-        timeout-minutes: 20
+        timeout-minutes: 25
         run: |
           make ${TEST}