cmd/addr2line: use the test binary as 'addr2line' instead of rebuilding it twice

cmd/addr2line: use the test binary as 'addr2line' instead of rebuilding it twice

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: Ie9a3c03d32d7eea268ba6f8f8ac4000539434052
Reviewed-on: https://go-review.googlesource.com/c/go/+/450713
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>