[Xcb] unit tests

[Vincent Torri]

Hey,

iirc, Bart talked about unit tests for xcb. As check is installed, and an 
example exists, it should be easy to add more tests.

So, maybe we should write a document in which we add all the tests we want 
for xcb.

also, using gcov et lcov for an html output may help us in detecting which 
functions is not yet covered by the unit tests

also, maybe we could write some unit tests for the protocol itself, using 
the xml description.

any other ideas ?

Vincent