Include Yosys in VtR test infrastructure

[acomodi]

As SymbiFlow heavily depends on Yosys it would be useful to integrate it in the VtR testing infrastructure to let it possible having SymbiFlow tests being run directly in VtR.

Todo

• Check how the testing infrastructure is built to easily inject Yosys in the flow
• Try to use Yosys in the flow with a simple test
• Make a mergable patch that includes Yosys in VtR