Unfortunately, WordPress does not yet have a good way for you to reference common files between plugins. In the case of these tests, it would be more efficient to put them into some sort of global WordPress library so that multiple plugins could use the tests contained in a single testing class file. However, until our testing scripts get integrated into the WordPress core, you have to work with a couple of options.
The first option is that you can copy all your testing functions into a dedicated class used by only your plugin (as we did in the previous examples). The upside here is that this is guaranteed to work, but the downside is that it's inefficient—imagine if 100 plugins each copied the exact same tests. It would take up 100 times more space, and it would be a nightmare to maintain—even the most trifling change in one of the tests would require that you make the same alterations to the other 99 copies.
As part of a second option, you can put all of these tests...