Move "PROJECT_DIR/sideboard/tests" package to "PROJECT_DIR/tests"

[RobRuana]

This is largely a personal preference, and there are pros and cons to both directory structures.

Hopefully we can get the best of both worlds with the following configuration:

• PROJECT_DIR/sideboard/tests will contain utilities to facilitate test creation by both sideboard developers and plugin developers
• PROJECT_DIR/tests will contain the actual sideboard unit/functional tests

Since I am going to be working on tests anyway, I figured I may as well do this while I'm at it.