The goal here was to enforce 100% coverage on
parse_narrow, but the code has an unreachable line
and is overly tolerant of bogus urls. This will
be fixed in the next commit.
This run_test helper sets up a convention that allows
us to give really short tracebacks for errors, and
eventually we can have more control over running
individual tests. (The latter goal has some
complications, since we often intentionally leak
setup in tests.)