cmd/gorename: add tests that run successfully

Set the env variable to only include the correct gopath in the test.

Fixes golang/go#20805

Change-Id: I496ed5c60278e508016f915cf53a115cc3c222ae
Reviewed-on: https://go-review.googlesource.com/47410
Reviewed-by: Alan Donovan <adonovan@google.com>
1 file changed