all: make safe for new vet analyzer

all: make safe for new vet analyzer

The unused analyzer handles dot imports now, so a few tests
have picked up vet errors. This CL errors like:

context/x_test.go:524:47: result of context.WithValue call not used

Change-Id: I711a62fd7b50381f8ea45ac526bf0c946a171047
Reviewed-on: https://go-review.googlesource.com/c/go/+/493598
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>