patch 9.1.0937: test_undolist() is flaky

Problem:  test_undolist() is flaky
Solution: allow to match one additional optional whitespace char

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_undo.vim b/src/testdir/test_undo.vim
index 75ee4a5..844ba77 100644
--- a/src/testdir/test_undo.vim
+++ b/src/testdir/test_undo.vim
@@ -282,7 +282,8 @@
     w Xundolist.txt
     defer delete('Xundolist.txt')
     let lastline = execute('undolist')->split("\n")[-1]
-    call assert_match("ago        1", lastline)
+    call assert_match('seconds\? ago         \?1', lastline)
+
   endif
   bw!
 endfunc