runtime: check that pprof accepts but doesn't need executable

runtime: check that pprof accepts but doesn't need executable

The profiles are self-contained now.
Check that they work by themselves in the tests that invoke pprof,
but also keep checking that the old command lines work.

Change-Id: I24c74b5456f0b50473883c3640625c6612f72309
Reviewed-on: https://go-review.googlesource.com/37166
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>