Update the project documentation with the testing guidelines

As a reminder I suggest updating the project docs with these:

• each piece of logic being tested should live in a separate test function
• only parametrize tests if the same logic is tested with different input values
• prefer self-contained test functions with duplicated code if necessary (within reason)
• avoid test function with too much logic, that is overly modular / parametrized, or makes heavy use of fixtures or other setup code

Originally posted by @dime10 in https://github.com/PennyLaneAI/catalyst/pull/98#discussion_r1190445420

The closest known gudelines are https://github.com/PennyLaneAI/guidance-docs/blob/master/development/code-review.md#code-author-and-reviewers-checklist

Read the referenced development/code-review.md checklist and the linked pull-request discussion first. Locate the project's documentation area, then add the four testing-guideline points in the appropriate section and verify that the documentation clearly reflects the requested testing practices.