patch 9.0.0415: on MS-Windows some tests are flaky

Problem:    On MS-Windows some tests are flaky.
Solution:   Add sleeps, disable swapfile, mark test as flaky. (Ken Takata,
            closes #11082)
diff --git a/src/testdir/test_undo.vim b/src/testdir/test_undo.vim
index a2f3c01..61cab68 100644
--- a/src/testdir/test_undo.vim
+++ b/src/testdir/test_undo.vim
@@ -335,6 +335,11 @@
 endfunc
 
 func Test_undofile_earlier()
+  if has('win32')
+    " FIXME: This test is flaky on MS-Windows.
+    let g:test_is_flaky = 1
+  endif
+
   " Issue #1254
   " create undofile with timestamps older than Vim startup time.
   let t0 = localtime() - 43200