Add test for Go's verification code.

[agl]

This attempts to be a strict pass/fail test and thus it categorises each
of the “WEAK-OK” cases. That involves matching the description strings,
which is unfortunate but currently unavoidable.

It does not yet pass, but will on Go master once [1] has landed.

[1] https://go-review.googlesource.com/#/c/36900/

[JackOfMostTrades]

Thanks Adam. I've thought about doing a little bit of refactoring to define test expectations better depending on intended client behaviors (e.g. whether or not IP name constraints are enforced at all, or whether the client ignores CN entirely when SAN is present), but in the mean time matching description strings works for me.