vim-patch:9.1.0820: tests: Mac OS tests are too flaky (#30980)

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: vim/vim#15940

baab7c0865

Co-authored-by: Milly <milly.ca@gmail.com>
Co-authored-by: K.Takata <kentkt@csc.jp>

test/old/testdir/runtest.vim

@@ -174,10 +174,6 @@ func GetAllocId(name)
   return lnum - top - 1
 endfunc
 
-if has('reltime')
-  let g:func_start = reltime()
-endif
-
 " Get the list of swap files in the current directory.
 func s:GetSwapFileList()
   let save_dir = &directory
@@ -567,6 +563,16 @@ for g:testfunc in sort(s:tests)
   " A test can set g:test_is_flaky to retry running the test.
   let g:test_is_flaky = 0
 
+  " A test can set g:max_run_nr to change the max retry count.
+  let g:max_run_nr = 5
+  if has('mac')
+    let g:max_run_nr = 10
+  endif
+
+  " By default, give up if the same error occurs.  A test can set
+  " g:giveup_same_error to 0 to not give up on the same error and keep trying.
+  let g:giveup_same_error = 1
+
   let starttime = strftime("%H:%M:%S")
   call RunTheTest(g:testfunc)
 
@@ -582,10 +588,15 @@ for g:testfunc in sort(s:tests)
       call extend(s:messages, v:errors)
 
       let endtime = strftime("%H:%M:%S")
-      call add(total_errors, $'Run {g:run_nr}, {starttime} - {endtime}:')
+      if has('reltime')
+        let suffix = $' in{reltimestr(reltime(g:func_start))} seconds'
+      else
+        let suffix = ''
+      endif
+      call add(total_errors, $'Run {g:run_nr}, {starttime} - {endtime}{suffix}:')
       call extend(total_errors, v:errors)
 
-      if g:run_nr >= 5 || prev_error == v:errors[0]
+      if g:run_nr >= g:max_run_nr || g:giveup_same_error && prev_error == v:errors[0]
         call add(total_errors, 'Flaky test failed too often, giving up')
         let v:errors = total_errors
         break
@@ -596,7 +607,8 @@ for g:testfunc in sort(s:tests)
       " Flakiness is often caused by the system being very busy.  Sleep a
       " couple of seconds to have a higher chance of succeeding the second
       " time.
-      sleep 2
+      let delay = g:run_nr * 2
+      exe 'sleep' delay
 
       let prev_error = v:errors[0]
       let v:errors = []