Document guidelines for writing tests

[PatrickvL]

Describe how tests are supposed to be written. A few suggestions:

Test as much variation of input arguments as reasonably possible.
Test if wrong input is denied according to specs.
When arguments have to obey some rule that can't be tested, describe the reason for skipping that test in a comment.
Test extreme (min, max, magic) argument values.
Test that all known effects have happened.
Always cleanup afterwards.
The only allowed resource leaks are those that are impossible to avoid.