Fixed `displayed` chai assertion in test.rfb.js

The implementation of the `displayed` chai assertion from test.rfb.js
was overriding the one from test.display.js, which was the correct one
that took viewport location into consideration.

chai assertions should probably be extracted in order to reduce code
duplication and prevent similar errors in the future.

Call resize() on client._display instead of setting _fb_width and
_fb_height properties directly otherwise _viewportLoc width and height
are not properly set, causing tests to fail with "Disconnected (1
times), because no message in 10000 ms" error.