patch 9.1.0571: tests: Test_gui_lowlevel_keyevent is flaky

Problem:  tests: Test_gui_lowlevel_keyevent is flaky
Solution: mark as flaky (Ken Takata)

Test_gui_lowlevel_keyevent is failing on vim-win32-installer recently.
Mark it as flaky for now.

closes: #15223

Signed-off-by: Ken Takata <kentkt@csc.jp>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_gui.vim

@@ -1706,6 +1706,7 @@ endfunc
 func Test_gui_lowlevel_keyevent()
   CheckMSWindows
   new
+  let g:test_is_flaky = 1
 
   " Test for <Ctrl-A> to <Ctrl-Z> keys
   for kc in range(65, 90)

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    571,
 /**/
     570,
 /**/