Define macros gz_build_display_tests and gz_build_dri_tests. Issue #478
These macros are defined empty if not valid support is found.
Note that currently both of them are generating qt tests (reusing the old gz_qt_build_tests). I'm not sure if this will be correct for all the cases (i.e: tests needed display but not using qt). What do you think?
Sensors and Rendering tests do not need QT support.
Yes, we will have tests that need DRI but do not need a DISPLAY. It's ok to have a DISPLAY (it won't effect the tests), but it's not mandatory. I think it might be the case that a DISPLAY is required to get DRI.