Implement testing in CI pipeline using venv

[BenjaminRodenberg]

Due to legacy developments our tests in the CI pipeline are executed relying on system dependencies. This approach is problematic:

• Here we need to use the option --break-system-packages in order to install the fake preCICE package
• In e9fbeb3 the tests don't work since SciPy is not installed on the system. I manually installing SciPy via apt install python3-scipy fixes this issue and I'm planning to add this as a work around soon.

Note that working on #177 might also fix these problems. But I'm not very familiar with tox and some additional steps might be necessary. I am therefore opening this issue for a better separation of concerns.