PyTest extension for WPS testing?

We could write a `pytest` extension to support testing of `PyWPS` services but also for testing WPS clients, like `birdy`.

This extension could be a python package on its own ... but we might also like to have it as an extension of `PyWPS`, like the sphinx autodoc extension:
https://github.com/geopython/pywps/blob/master/pywps/ext_autodoc.py

See also related discussion in issue:
https://github.com/bird-house/birdy/pull/86

Start by reading the proposal and the referenced pywps/ext_autodoc.py extension, then review the related discussion in birdy/pull/86. Clarify whether the work belongs in a standalone Python package or as a PyWPS extension, and define support for testing PyWPS services and Birdy-like WPS clients before implementation.

Written by the indexing model from the issue text.