cmd/go: reroute vcs-test.golang.org HTTPS requests to the test-local server

cmd/go: reroute vcs-test.golang.org HTTPS requests to the test-local server

After this CL, the only test requests that should still reach
vcs-test.golang.org are for Subversion repos, which are not yet handled.

The interceptor implementation should also allow us to redirect other
servers (such as gopkg.in) fairly easily in a followup change if
desired.

For #27494.

Change-Id: I8cb85f3a7edbbf0492662ff5cfa779fb9b407136
Reviewed-on: https://go-review.googlesource.com/c/go/+/427254
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>