test: add litest_push/pop_event_frame() helpers

test: add litest_push/pop_event_frame() helpers

For some tests we need to string multiple event sequences together into one
event frame. Use a push/pop frame approach that stops litest from sending any
EV_SYN/SYN_REPORT events, so we can merge two touches together by e.g.

litest_push_event_frame(d);
litest_touch_down(d, 0, 10, 10);
litest_touch_down(d, 1, 20, 50);
litest_pop_event_frame(d);

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>