runtime/pprof: print stderr on test failure

runtime/pprof: print stderr on test failure

Print Stderr on test failure to track down the intermittent
test failure reported in issue #62352.

Change-Id: I547a3220dc07d05578dac093d6c028a9103b552a
Reviewed-on: https://go-review.googlesource.com/c/go/+/524156
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>