This fixes a bug related to shutting down of GRPC plugin interfaces
(more info: https://github.com/hashicorp/go-plugin/pull/88)
This does not yet fix all test cases for subprocess leaking, but is a
useful independant change.
This drops the testings stdlib pkg from our dependencies. Saves a
whopping 46kb on our binary (was really hoping for more of a win there),
but also avoids potential ugliness with how testing sets flags.
