add SILENT_ENSURE macro

[felipecruz]

I'm testing a generator like function, that yields char by char and I felt the need to SILENT_ENSURE (c == b) instead of ENSURE which prints #expr every time. I know thc has a non verbose mode but in this case I do want verbose output with specific silent ensures.

[hltbra]

What's the reason to have some tests with output and others with no output? What's the point here? Have you thought about this silent thing being an option to thc_run(), like thc_run(THC_SILENT)?

(Sorry for taking so long, @felipecruz, but I was in a hurry because I moved to NYC)