Make the StackCopyNoCache test easier to read.

Add a StackCopyPtr test that actually has some pointers
that need adjusting.

Change-Id: I5b07c26f40cb485c9de97ed63fac89a9e6f36650
Reviewed-on: https://go-review.googlesource.com/104195
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>