Add a new method TB.Setenv that'll set environment variables
only for the isolated lifetime of the test, and will clean up
and unset these variables when the test ends.
This method disables the test or benchmark from running in
parallel.

Fixes #41260

Change-Id: I0a18f094ec1c6ec3157b4b12993ea3075e2e9867
GitHub-Last-Rev: 0ca12fa565318f350b927e2ef94f3b4f792c75c2
GitHub-Pull-Request: golang/go#41857
Reviewed-on: https://go-review.googlesource.com/c/go/+/260577


Trust: Daniel Martí <mvdan@mvdan.cc>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: roger peppe <rogpeppe@gmail.com>