go/types, types2: better variable names, cleanups in test

go/types, types2: better variable names, cleanups in test

For #54258.

Change-Id: Ib0d326af2719bca1579f84c125f6573f87dce982
Reviewed-on: https://go-review.googlesource.com/c/go/+/452455
Run-TryBot: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>