Document the writing of tests

In GitLab by @timothy-nunn on Sep 17, 2021, 11:24

Testing is a critical part of any software, especially complex numerical codes such as PROCESS. A good practice in development is to continually create tests for new changes. It has come to my attention that the process for writing new tests may be slightly unclear. The writing of through tests is something we want to encourage and making the process easier would be positive.

I believe this documentation could come in with my updated "best practices" documentation for f2py wrapping in #1397 coming in as #1398.

• How to write different types of tests
• The distinction between different types of tests
• Good testing practices

@cz1743 Anything missing from the list above? Anything else you find in your test writing that you think could be (better) documented just comment below.