runtime/pprof: fix memory profiler test
Don't use cmd/pprof as it is not necessary installed
and does not work on nacl and plan9.
Instead just look at the raw profile.

LGTM=crawshaw, rsc
R=golang-codereviews, crawshaw, 0intro, rsc
CC=golang-codereviews
https://codereview.appspot.com/159010043