Move tests to dedicated directory

[wllgrnt]

There are philosophical reasons you might not wish to do this, but it is more standard for python packages and has advantages e.g in not packaging the tests - if we take this approach, this is what needs to change to make it so.

How Has This Been Tested?

pytest

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.