cmd/go: update test for change in panic(nil) behavior

cmd/go: update test for change in panic(nil) behavior

panic(nil) now panics with runtime.PanicNilError.

For #25448

Change-Id: I58994aa80d4d11f0c5fcd988714f7b4b45c6b5ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/462293
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>