go/types, types2: consolidate testdata/spec test files

go/types, types2: consolidate testdata/spec test files

Besides applying gofmt (which doesn't damage the tests in this case),
this removes a TODO for a set of nil-related tests.

The test files are now identical.

For #54511.

Change-Id: I0a286ad607f317f43972c1f5ee741b4f5bc9576d
Reviewed-on: https://go-review.googlesource.com/c/go/+/424675
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>