[release-branch.go1.12] cmd/go/internal/{modconv,modfetch,modload}: set modfetch proxy URL in tests

Updates #30571
Fixes #34789

Change-Id: Id4c74e83ee58a080d1c2894ae5ebdbf4aeb1ce42
Reviewed-on: https://go-review.googlesource.com/c/go/+/167084


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit bd680d94)
Reviewed-on: https://go-review.googlesource.com/c/go/+/200037