[[ https://jenkins.softwareheritage.org/job/DGRPH/job/tests/ | CI builds ]] for swh-graph were failing since a couple of weeks, likely due to some recent changes in upstream pytest.
That diff fixes pytest invocation in tox.ini.
The `tests/dataset` folder is also moved in `swh/graph/tests` as it is required to run the tests.
Previously it was installed in `<python-site-packages>/tests` which is not a proper install location.
Related D2100