test-compositor: add display_send_data and client_recieve_data

Using these function the programmer can send data from display to
client.
Tests for the function added as well
test-compositor: add DBG_LINE macro

This macro only prints information about the line it was called on.
It's usefull for debugging concurrency in tests
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: fix bad arguments when posting event

I was getting SIGSEGV in resources-test (arch linux, gcc 4.8.2) due
to bad (no) arguments passed to wl_resource_post_event().
Although the event is not invoked it crashed in serialize_closure
when handling arguments
tests: add wl_display tests

Test what is written in documentation
fixed-benchmark: remove warning

Compiler warned about unused arguments and it was right