test: switch from nose2 to pytest

[eginhard]

Switch to a modern testing framework that enables the following:

• use temporary output folders to not pollute the user space
• use @pytest.mark.parametrize where applicable to reduce duplication
• switch to pytest fixtures for easier setup
• run CLI tests via Python to get test coverage from them
• reduce duplication in model training tests

Fixes coqui-ai#3192