Move Sleep process definition

[huard]

Overview

Moves the Sleep process class definition with other test processes to facilitate reuse.

Related Issue / Discussion

In response to comments in #657

Additional Information

Contribution Agreement

(as per https://github.com/geopython/pywps/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to PyWPS. I confirm that my contributions to PyWPS will be compatible with the PyWPS license guidelines at the time of contribution.
• I have already previously agreed to the PyWPS Contributions and Licensing Guidelines

[coveralls]

Coverage increased (+0.03%) to 81.087% when pulling 53ba8ac on huard:refactor_sleep_test into 85ca819 on geopython:main.

[gschwind]

Hello,

Look good to me, maybe I will use Sleep._handler instead of self._handler but it's fine like it.

I did look at code and run pytest in my environment for this review.

Best regards.

[cehbrecht]

@huard looks good. Thanks :)