Use xdist for functional tests

[mkniewallner]

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tartiflette currently depends on pytest-xdist for test dependencies, but doesn't make use of it.
Passing -n auto to pytest for functional tests speeds up test durations both locally and on the CI.
As functional tests are isolated, it shouldn't cause any issue.

For unit tests, they are already rather fast, so it is not worth IMO to use multiple processes.