added sim.log in /sim_build directory

[gretzteam]

Everything showing up in the terminal when running a simulation isn't part of any log files when using cocotb-test.
This includes all the log messages from cocotb, such as:
self.dut._log.info("Doing something")

The following changes to the Simulator class enable that: sim.log is created in the /sim_build directory.

[themperek]

It is not better to use the standard way to save logs to files provided by pytest/python rather than add something custom?

See: https://docs.pytest.org/en/stable/logging.html

[themperek]

My plan is to recognize logging from cocotb/simulator and properly forward it python logging but need some time to do this (with INFO/WARNING etc).

[gretzteam]

Implementation using python logger in #97