Adding C++ unit tests to Zeek

We're thinking to add lower level C++ unit tests into Zeek to more
easily cover code that's otherwise hard to test since you have to
reach it indirectly via a script-layer test in the existing
baseline-oriented test suite.

Let us know if there's any input on choice of unit test framework or
features/requirements that are desirable. I started with my own
thoughts here:

- Jon