pytest `--forked` option is not supported

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1111)
In GitLab by [[Gitlab user @willsalmon]](https://gitlab.com/willsalmon) on Aug 28, 2019, 15:30

## Summary

Using the `--forked` option of pytest causes many tests to fail.

## Steps to reproduce

Try running:

```sh
tox -e py38-nocover -- --forked tests/artifactcache/config.py::test_only_one[storage-project]
```

This fails, while running:

```sh
tox -e py38-nocover -- tests/artifactcache/config.py::test_only_one[storage-project]
```

Does not fail.

We can see that the test passes fine when run individually, but fails if it is given the problematic `--forked` option.

Start by running the reported tox command for tests/artifactcache/config.py::test_only_one[storage-project] with and without --forked, then inspect the test and its surrounding artifact-cache setup. Done means the test passes when invoked through tox with --forked, without regressing the non-forked run.

Written by the indexing model from the issue text.