cmd/objdump: ensure that test executable files are distinct
cmd/objdump: ensure that test executable files are distinct

This fixes test failures introduced by CL 176901.

Change-Id: I133299ba3be3a15ced076c95e4833ba6070d7eb7
Reviewed-on: https://go-review.googlesource.com/c/go/+/176903
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/go,CAFxX/go,nawawi/go,akutz/go,cloudflare/go,nawawi/go,CAFxX/go,cloudflare/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,golang/go,golang/go,akutz/go,akutz/go,cloudflare/go,nawawi/go,cloudflare/go,nawawi/go,cloudflare/go,CAFxX/go,CAFxX/go,golang/go,golang/go,nawawi/go,akutz/go,akutz/go,akutz/go,CAFxX/go,CAFxX/go,akutz/go,akutz/go,golang/go,golang/go,CAFxX/go,cloudflare/go