Fix tox execution and move dataset folder
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 !185 (closed) (not fixed either)
Migrated from D2176 (view on Phabricator)