[dev.typeparams] cmd/compile/internal/importer: review of gcimporter_test.go

This CL removes the // UNREVIEWED disclaimer at the top of the
file. This file is essentially a copy of its reviewed version
at src/go/internal/gcimporter/gcimporter_test.go with adjustments
to make it work for the compiler and types2. To see the changes
made with respect to the original, compare patchset 2 against
patchset 3.

Change-Id: Iaeb9a56a6a56f4c1d93e7bfedc5b1f1968fa6792
Reviewed-on: https://go-review.googlesource.com/c/go/+/324131


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>