cmd/go: declare net hosts in script tests

cmd/go: declare net hosts in script tests

Although we aren't precise about enforcing the hosts just yet,
we can eventually use the declared hostnames to selectively skip
tests (for example, if an external service has an outage while
a Go release is being tested).

Also relax the constraint to [short] in tests that require only
vcs-test.golang.org, which has redirected to an in-process server
since around CL 427914.

Also enforce that tests that use the network actually use the [net]
constraint, by setting TESTGONETWORK=panic in the test environment
until the condition is evaluated.

For #52545.
For #54503.
Updates #27494.

Change-Id: I13be6b42a9beee97657eb45424882e787ac164c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/473276
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Bypass: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>