cmd/go: stream test output if parallelism is set to 1.

"go test -v" buffers output if more than one package is
being tested to avoid mixing the outputs from multiple
tests running in parallel. It currently enables streaming
if there's only a single package under test.

It is ok to stream output from multiple tests if we know
that they're not going to be running in parallel.

To see the difference: go test -v -p=1 runtime fmt -short

Change-Id: Idc24575c899eac30d553e0bf52b86f90e189392d
Reviewed-on: https://go-review.googlesource.com/4153


Reviewed-by: Ian Lance Taylor <iant@golang.org>