Allow SLAssert methods to be called from non SLTest subclass methods

What do you think about this? I was planning on building a library of helper methods to build up the test cases, but unless they subclass SLTest, I can't call the assert methods. This doesn't seem right to me. 

This is my proposed solutions, but I wanted to get some thoughts.
Let's move this sentence and the @warning below into a comment right below the pragma, not tied specifically to `SLAssertTrue`.