This is needed to handle vendor directories correctly.  It was already
done for the regular imports when the package was loaded, but not for
the test-only imports.

It would be nice to do this while loading the package, but that breaks
the code that checks for direct references to vendor packages when
running go test.  This change is relatively contained.

While we're at it, skip "C" test imports in go get.

Fixes #11628.
Fixes #11717.

Change-Id: I9cc308cf45683e3ff905320c2b5cb45db7716846
Reviewed-on: https://go-review.googlesource.com/12488
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>