display-test: make use of create_thread function

This function is used in one test only, but its functionality can be
used in another tests to (create thread and wait until it is sleeping).
We just need to pass the starting function for the thread as an argument.

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>