Set $GOPATH to a semantically valid, non-empty string that cannot
conflict with $GOROOT to avoid false test failures that occur when
$GOROOT resides under $GOPATH.  Unsetting GOPATH is no longer viable
as Go now defines a default $GOPATH that may conflict with $GOROOT.

Fixes #19237

Change-Id: I376a2ad3b18e9c4098211b988dde7e76bc4725d2
Reviewed-on: https://go-review.googlesource.com/37396Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>