cmd/compile/internal/types2: review of importer_test.go

This is a small helper file that provides a default importer
for the type checker tests. There is no go/types equivalent.

The actual change is removing the "// UNREVIEWED" marker.

Change-Id: Ic1f9858bdd9b818d9ddad754e072d9d14d8fb9b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/304252


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>