misc/cgo/testshared: make -v output less verbose

Previously, 'go test -v' in this directory would result in a massive
dump of go command output, because the test plumbed -v to 'build -x'.
This change separates them into distinct flags, so that '-v' only
implies the display of default 'go' command output.

Updates #30316

Change-Id: Ifb125f35ec6a0bebe7e8286e7c546d132fb213df
Reviewed-on: https://go-review.googlesource.com/c/go/+/208232


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>