use VCL_HIDE_WINDOWS env on Mac to 'simulate' headless mode on Mac

Building, and running the unit/subsequent test on Mac cause massive amount
of flashing windows creation/destruction that make building with test
very annoying on a box used for other purposes.
This patch prevent the windows to be actually displayed hence mitigating
the problem

Change-Id: Ied2a36699b50856e0c4e6b618212481796ec99d3