Tests: Add time emulation feature for tests with BEREMIZ_TEST_CYCLES CFLAG.

Adding BEREMIZ_TEST_CYCLES=1000 in a project's CFLAGS will:
- run 1000 cycles with no pause
- emulate time flowing normaly for PLC code
- exit PLC thread

This allows:
- testing standard library blocks that deal with time without having to wait
- unit testing and code coverage with POUs that uses time

.. Beremiz documentation master file

Beremiz's documentation
=======================

Contents:

.. toctree::
   :maxdepth: 2

   overview
   manual/index
   standards