patch 9.1.0587: tests: Test_gui_lowlevel_keyevent is still flaky

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>