[release-branch.go1.20] all: make safe for new vet analyzer

[release-branch.go1.20] 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

This is a manual cherry-pick of CL 493598.

Updates #60058
Fixes #60927

Change-Id: I92906ef7967e14a85fa974e6307fd689e3ff3dba
Reviewed-on: https://go-review.googlesource.com/c/go/+/504977
Auto-Submit: Heschi Kreinick <heschi@google.com>
TryBot-Bypass: Heschi Kreinick <heschi@google.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>