Proposal: clean up the examples and tests

[riccardobl]

As we've been consistently adding tests for issues within the jme3-examples, I realize this might be confusing for newcomers using the examples to learn about the engine.
My proposal is to relocate all tests, including unit tests and assets, into a dedicated jme3-tests module.

What do you think?

[stephengold]

I agree this ought to be done.
Please see previous discussion at #1290.