prune: add ability to strip tests

[adg]

It would be great if dep prune would strip tests.

Currently my update process is:

    $ dep ensure -update
    $ dep prune
    $ find vendor -name '*_test.go' -delete

It'd be nice to roll the last command into prune, especially since prune already strips testdata directories.

[sdboyer]

yep, definitely should have this. i need to write up an issue specifically outlining how to move prune back into ensure.

[sdboyer]

now that we have #944, closing this in favor of that 😄