tests: check wl_display_roundtrip() for errors

tests: check wl_display_roundtrip() for errors

Add a macro that wraps wl_display_roundtrip() and check for errors. It
is a macro, so that the assert would show the relevant file and line
number.

This will also catch protocol errors, that would go unnoticed otherwise.

All roundtrips in tests are replaced with the check.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>