test: fix -test.v trace output for cgo/testshared

test: fix -test.v trace output for cgo/testshared

Trace output showing how dummy GOROOT was being set up was incorrect
(sense of the "cp -r" trace messages was inverted). This patch fixes
the problem.

Change-Id: Ib0ee649e305bfa1bc0c49e0d5ba2ea31e0a4f67e
Reviewed-on: https://go-review.googlesource.com/c/go/+/224377
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>