Communicate "why it matters" on each testcase

[woodruffw]

Pointed out by @reaperhulk: the current metadata for each testcase communicates whether it passes or fails, but doesn't adequately communicate why or how severe a disagreeing result would be.

We should figure out some way to communicate that, e.g. "failing this testcase means that an implementation is likely to accept certificates for unintended domains."

[woodruffw]

Thinking a little more about this: we could probably have a Severity enumeration that defaults to UNDETERMINED, and goes UNDETERMINED, LOW, MEDIUM, HIGH, CRITICAL.

[woodruffw]

#236 adds the Importance model and qualifier to each testcase. Leaving this open for the actual fill-in.