cmd/compile: clean up debug_test.go

cmd/compile: clean up debug_test.go

Exercise of preparing a how-to document motivated me to
clean up some of the stupider wonkier bits.  Since this
does not run for test -short, expect no change for trybots,
did pass testing with OSX gdb and a refreshed copy of Delve.

Change-Id: I58edd10599b172c4787ff5f110db078f6c2c81c5
Reviewed-on: https://go-review.googlesource.com/83957
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>