This allows us to run the tests via a library call rather than just
as a command execution. And, it's a step toward a broader goal, which
is to keep the code in the top-level src directory minimal, with
important functionality exposed as crates.
Change the default value for the "enable_verifier" setting so the
verifier runs unless it is explicitly disabled.
Most projects using Cretonne are best off running the verifier always
until they start caring about compile time performance. Then they can
easily disable the verifier.
