cmd/objdump: use the test binary as 'objdump' instead of rebuilding it

cmd/objdump: use the test binary as 'objdump' instead of rebuilding it

This not only reduces the latency of the test, but also respects
build flags like '-race' and '-cover' passed to the 'go test' command.

Change-Id: Icd22062ab75964a74d011c81ea6c99be80bece18
Reviewed-on: https://go-review.googlesource.com/c/go/+/450704
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>