Add SlapOSDoc test suite class to tests/__init__.py

We add a new test suite to run the test pages in the erp5-doc repository:

https://lab.nexedi.com/nexedi/erp5-doc/blob/master/bt5/erp5_slapos_tutorial/PathTemplateItem/test_page_module/slapos-Tutorial.Install.Slapos.Master.Comp.Root.html

PS: The test page still WIP.

Put this test suite in this repository is because the test pages in the erp5-doc/erp5_slapos_tutorial need software/slapos-master/software.cfg recipe to configure the erp5 instance.

And in this recipe, the tests/__init__.py of the slapos.core repository has the highest priority: https://lab.nexedi.com/nexedi/slapos/blob/master/software/slapos-master/software.cfg#L27