Fix file paths in tests

[epassaro]

Describe the bug

It's not possible to test the tardis package once installed by doing pytest --pyargs tardis. This happens because some yml have their paths hardcoded in the test files.

This is particularly important to test the conda-forge package release.

To Reproduce

pytest --pyargs tardis

See: https://docs.pytest.org/en/6.2.x/example/pythoncollection.html#interpreting-cmdline-arguments-as-python-packages

[rohithvarma3000]

@epassaro, I am not able to understand the exact issue. I can run the command without problems(except for some failing tests). Could you retry this command and let me know the error you are facing?