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

cmd/pprof: use the test binary as 'pprof' 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: Icdf256420c4dce2da7a187513b7dd08393b76146
Reviewed-on: https://go-review.googlesource.com/c/go/+/450708
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>