tests: Add test for keyboard key event timestamps

tests: Add test for keyboard key event timestamps

Add test to verify that the server correctly sets the timestamps of
keyboard key events. This requires updating the weston-test protocol to
support passing key event timestamps.

simple_keyboard_test now uses the create_client_with_keyboard_focus()
helper function which changes the initial state of the surface to be
focused. This leads to one additional iteration of the test loop when
starting, during which the surface is deactivated, i.e., loses focus.
After this initial iteration the test continues as before.

Furthermore, simple_keyboard_test now uses the send_key() helper
function which performs a roundtrip internally. To account for this, the
client_roundtrip() function is now directly called in the loop only when
it is still required, i.e., when deactivating the surface.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>