add tests folder and basic test

[tim-werner]

Add test folder and basic test to the cookiecutter template.

[jamesmyatt]

When adding tests outside of the application, it's usual to either avoid making tests a module or to use a src layout. See https://docs.pytest.org/en/latest/goodpractices.html#tests-outside-application-code

[pjbull]

Closing since I think we will reconsider options for this after #162 is in. Issue filed #195