test-compositor: add client_deinit function

This function should be used to free resources after client_init
function is called. Now it only calls display_disconnect, but ut's more consistent
that to do it manually and it can be extended in the future.
tests: add wl_display tests

Test what is written in documentation
test-runner: add zero_config

Display is frequently created with config {0, 0, 0}. Add this
configuration as global zero_config variable so that user doesn't need to create
it all the time.
Note: it has been added into libtc-utils (not libtc-server) because I
plan to extend config into client too.
test-compositor: add const to display_create() param

The config parameter is not modified anywhere in the function(s)