tests/aio: Clean up global variable shadowing

tests/aio: Clean up global variable shadowing

Rename the argument to fix:

  tests/unit/test-aio.c:130:44: error: declaration shadows a variable in the global scope [-Werror,-Wshadow]
  static void set_event_notifier(AioContext *ctx, EventNotifier *notifier,
                                             ^
  tests/unit/test-aio.c:22:20: note: previous declaration is here
  static AioContext *ctx;
                     ^

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20231009100251.56019-9-philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>