functest package

[bradfitz]

I've pushed an initial draft of a "functest" package to:

https://godoc.org/go4.org/testing/functest

Feedback welcome here in this issue on its API, implementation or feature requests.

/cc @mpl

[mpl]

I'm not sure yet about the usefulness of the whole thing, but I can see at least a win from easy naming of test cases and the resulting error message formatting.

a couple of typos I've noted : "res.Paniked", "whether the function paniced", "Callers my use both".