The test is only defined on darwin/amd64, and it fails
with recent versions of Xcode, which do not support
-gstabs+ debugging output.  At some point godefs will
have to be replaced, perhaps merged with cgo.
Godefs is not needed during builds anyway (its output files
are checked into the repository in src/pkg/runtime),
so its failure on the newer Xcode is a distraction from an
otherwise usable build.  Disable the test.

Fixes #1985.

R=golang-dev, gri, robert.hencke, r
CC=golang-dev
https://golang.org/cl/4638053