Annoying graphics tests taking over focus

[rbouqueau]

For the record: while @soheibthriber and @nlsdvl made tests on tests, they complained (iirc) that tests would open graphical windows that change the focus. This prevents executing the testsuite while doing something else. The problem seems more annoying on MacOS (old window don't always regain focus while it does on Linux+Windows).

I had suggested to make these tests "second class", which had been rejected. Open to new suggestions.