CI/CD Testing: compiling 3rd party software on Github

[varisd]

We need to add logic for preparing 3rd party dependencies (Marian NMT, OpusCleaner Python virtualenv, etc) on github for automated testing.
I am proposing following:

• each dependency will be prepared in a fixed-name directory,
• in tests/conftest.py, we will have session-scope fixtures for each dependency which will check whether the software is present/available (otherwise it will skip the fixture and the dependent tests) and return their respective directories (usable in subsequent tests)

Or do we have a better solution for these scenarios?