Test bench in cocotb

[tigitadmin]

Add a test bench based on cocotb. Maybe as replacement for the avocado based test bench.

[tmahlburg]

This is mostly done, although the verilog testbenches have been ported quite directly, so there might be some potential optimizations. The following points still need work:

• finish PLLE2_ADV cocotb testbench
• create a Makefile (or multiple) to run all testbenches at once and to emulate the functionality of the avocado tests using the provided verilog wrappers. This can be done by setting different configurations using the exposed macros.
• rewrite GitHub Actions to use cocotb instead of avocado

The current work can be found in the linked pull request and in the cocotb branch.