Adding regression tests to rcov

I just began to eat my own dog food, applying rcov to its own regression tests. The coverage figure is still pathetic, but at least it's going up: rcov had been essentially untested (or only tested manually, which amounts to about the same) for way too long time. The main reason was bad laziness, and the self-justification that it's not easy to test code that relies on set_trace_func. Which was of course to a large extent false.

So here I am, adding regression tests semi-automatically with my xmp filter:

I now feel much more confident hacking rcov, and I finally got around to refactoring a bit. 0.4.0 is surely going to be quite cleaner and more featureful that 0.3.0.