cmd/objdump: mark tests as parallel

cmd/objdump: mark tests as parallel

Speeds up

go test -short -count=1 cmd/objdump

on my machine from 1.7s to 1.3s.

Not much, but as the backpacking saying goes,
take care of the ounces and the pounds will take care of themselves.

Updates #26473

Change-Id: I59fe9a179e48537c7d82cbba72cde9f92b42a029
Reviewed-on: https://go-review.googlesource.com/c/go/+/176901
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>