c-mode classically uses ff-find-other-file to toggle between headers
and implementation. For Go it seemingly makes sense to jump between
implementation and test.

While there's no enforced mapping of file names for tests, the mapping
in this CL seems to be very common at least throughout the standard
library, and ff-find-other-file fails gracefully when the mapping
doesn't apply.

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/65750044