patch 9.0.0426: failed flaky tests reports only start time

Problem:    Failed flaky tests reports only start time.
Solution:   Also report the end time.
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index 72a8f98..68c3dd3 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -479,7 +479,8 @@
       call add(s:messages, 'Found errors in ' .. g:testfunc .. ':')
       call extend(s:messages, v:errors)
 
-      call add(total_errors, starttime .. ' Run ' .. g:run_nr .. ':')
+      let endtime = strftime("%H:%M:%S")
+      call add(total_errors, $'Run {g:run_nr}, {starttime} - {endtime}:')
       call extend(total_errors, v:errors)
 
       if g:run_nr >= 5 || prev_error == v:errors[0]