patch 9.1.0587: tests: Test_gui_lowlevel_keyevent is still flaky

Problem:  tests: Test_gui_lowlevel_keyevent is still flaky
          (after v9.1.0571)
Solution: skip generating key event for Ctrl-C,
          remove the test_is_flaky variable again (Ken Takata)

v9.1.0571 was not enough.
Ctrl-C interruption may occur before the preceding events are processed.
Exclude Ctrl-C to avoid the flakiness.

closes: #15224

Signed-off-by: Ken Takata <kentkt@csc.jp>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_gui.vim b/src/testdir/test_gui.vim
index eefebff..ae65310 100644
--- a/src/testdir/test_gui.vim
+++ b/src/testdir/test_gui.vim
@@ -1706,10 +1706,10 @@
 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)
+  " FIXME: <Ctrl-C> is excluded for now.  It makes the test flaky.
+  for kc in range(65, 66) + range(68, 90)
     call SendKeys([0x11, kc])
     try
       let ch = getcharstr()