cmd/go: apply import restrictions to test code too

cmd/go: apply import restrictions to test code too

We reject import of main packages, but we missed tests.
Reject in all tests except test of that main package.

We reject local (relative) imports from code with a
non-local import path, but again we missed tests.
Reject those too.

Fixes #14811.
Fixes #15795.
Fixes #17475.

Change-Id: I535ff26889520276a891904f54f1a85b2c40207d
Reviewed-on: https://go-review.googlesource.com/31821
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>