rsc.io/toolstash is gone; use rsc.io/pprof_mac_fix.

This fixes a bug in the test. It turns out the code being tested here
is also broken, so the test still doesn't pass after this CL (filed #19769).

Change-Id: Ieb725c321d7fab600708e133ae28f531e55521ad
Reviewed-on: https://go-review.googlesource.com/38743Reviewed-by: Alan Donovan <adonovan@google.com>