Add cocotb verification environment

[koluckirafal]

This PR extends the verification environment and adds example test written using cocotb library.

└─ verification
   ├── test_cocotb
   ├── test_dev

In the directory test_cocotb you find a Python coroutine, which calls the top-level testbench testbench/tb_top.sv and simulates execution of a program. The core can be fed programs from external sources, e.g. RISC-V DV. To execute the test, a recipe in tools/Makefile is prepared.
In the directory test_dev you find an ALU test, which serves as an example and a starting point for developing rapid block-level unit tests, if needed.

[mkurc-ant]

Superseeded by PRs with microarchitectural tests. Closing.