Change test framework to use exit codes rather than inspecting stderr

[Rewbert]

The testing framework currently inspects stderr to figure out if running the c-code went alright, but we might want to use this stream for logging down the road. We should change the testing framework to use error codes instead and not rely on the stderr stream.

[j-hui]

We should use our own assertions (e.g., DEBUG_ASSERT) rather than relying on the C stdlib's assert macro---it gives us more control over what information to report, where to report it, and what to do afterwards.