Follow-up from "Added tests for diff analyzer"

The following discussion from !18 (merged) should be addressed:

@marius started a discussion:
Not part of this MR but I recommend to use the pytestconfig fixture which contains the rootdir which can be used e.g. here to get absolute paths to files.

Just as an incentive how to make this more flexible. Checkout https://docs.pytest.org/en/6.2.x/customize.html on 'finding the rootdir':
The internal Config object (accessible via hooks or through the pytestconfig fixture) will subsequently carry these attributes:
```
config.rootpath: the determined root directory, guaranteed to exist.

config.inipath: the determined configfile, may be None (it is named inipath for historical reasons).
```