test-compositor: add data element into struct client

In lot of tests, we need some data to be passed
to some function. We can create new structure that will held
all what we need, but sometimes it is just... too much of work.
Sometimes it is sufficient to store some data in client struct.

This patch adds pointer to user data to struct client.

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
display-test: use client->data instead of client->display_stopped

We used display_stopped to indicate that thread is sleeping. It was
comfy because we didn't have to create some helper variables. The
meaning was not clear, though. Now, when we have client->data element,
we can set the indicator variable as user data and use it instead.

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>