fix problem happend when tests are running under python-coverage

$ python-coverage run ./tests/tools/test_application.py

Exception while connecting LOCAL://!
Traceback (most recent call last):
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 1753, in _Connect
self._SetConnector(connectors.ConnectorFactory(uri, self)
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 1702, in _SetConnector
if connector is not None:
PyAssertionError: C++ assertion "m_window" failed at ../src/gtk/dcclient.cpp(2043) in DoGetSize(): GetSize() doesn't work without window

Project overview
================

In order to target the widest possible range of programmable devices and keep efficient, Beremiz use C code as an intermediate language. 

To be executed, C needs to be compiled. `GCC <http://gcc.gnu.org>`_ serve that purpose perfectly.

PLC program is expressed in languages defined in IEC-61131, including graphical languages. Thanks to PLCopen TC2, those graphical languages have a standardised representation, in XML.

To be continued.