go/types: migrate importer usage in tests

go/types: migrate importer usage in tests

To use an importer that knows where to find stdlib .a files based on
their new locations once the checked-in .a files are removed.

Change-Id: I981812306b3512380d58cb0f599a9a61b27ba0d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/442695
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>