Add a section on writing effective tests to the contributor guide

### Is your feature request related to a problem or challenge?

The existing contributor guide's "Testing" section only lists the existing test infrastructure: https://datafusion.apache.org/contributor-guide/testing.html

It would be helpful to add a section on how to write good tests, explaining the mental model and philosophy behind testing — something like "covering edge cases and error cases".

### Describe the solution you'd like

_No response_

### Describe alternatives you've considered

_No response_

### Additional context

_No response_