document how to run tests without venv nor hatch

[josch]

It is possible to run bmaptool and the testsuite without venv, without hatch, without pip and without building and installing anything into venv by just running this:

PATH=tests/bin:$PATH PYTHONPATH=src python3 -m unittest -bv

This assumes that a tool called bmaptool is in $PATH and essentially runs python3 -m src.bmaptool.

I find this way of running the tests quite a bit easier than going through all the venv, building and installation steps.