cmd/coordinator: disable OS X test sharding for now
cmd/coordinator: disable OS X test sharding for now

Workaround until the Mac failures are understood.

Updates golang/go#12979

Change-Id: I15b9ea8f4b708ebf9b7c6ad61e65d0f9eaaa6d73
Reviewed-on: https://go-review.googlesource.com/16440
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>

golang/build,proppy/build,golang/build,proppy/build,golang/build,proppy/build,golang/build,proppy/build