Add documentation for writing plugin tests

[elazar]

This should probably be added to this file. Thanks to @elstamey for the suggestion! :)

/cc @svpernova09 @matthewtrask

[elstamey]

As I said on Twitter, I'll write it if someone helps me understand how to do it.

[elazar]

We may want to consider having a separate repo that contains testing resources. This could pull in PHPUnit and Phake, provide a base test case class with helper methods for mocking events and the event queue, etc.

[elstamey]

I would guess that the plugin tests are written differently, in general. And it makes sense to have the resources with the plugin, imo.

[matthewtrask]

I may time this weekend to actually do this.