internal/testenv: add GOROOT and use it to fix tests broken with -trimpath

internal/testenv: add GOROOT and use it to fix tests broken with -trimpath

This fixes many (but not all) of the tests that currently fail
(due to a bogus path reported by runtime.GOROOT) when run with
'go test -trimpath std cmd'.

Updates #51461

Change-Id: Ia2cc05705529c4859e7928f32eeceed647f2e986
Reviewed-on: https://go-review.googlesource.com/c/go/+/391806
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>