Adding testing information to CONTRIBUTE

[Samer Masterson]

Hey list,

As a new contributor, I have no idea how to run or write tests for Emacs, which means I end up testing my patches manually and miss bugs. I could try to learn how Emacs uses tests on my own, but it would benefit future new contributors if we added a section on testing to CONTRIBUTE.

Here is what I would want to see:

 - How to write tests, with a pointer to an internal Emacs package that has idiomatic and comprehensive tests.

 - How to run tests on the entire tree and on individual files.

 - A pointer to an info page on the testing framework/pattern that most internal Emacs packages use.

Would anyone be interested in working on this? cc'ing Stephen Leake because he lead the most recent major changes to CONTRIBUTE.

-samer