Revert "testing: only compute b.N once when passed -count > 1"

Revert "testing: only compute b.N once when passed -count > 1"

This reverts golang.org/cl/110775

Reason for revert: this is causing huge slow-dows on every run after
the 1st, on various benchmarks, on multiple architectures (see Issue
25622 for details). It's just a nice-to-have little optimization, and
we're near the 1st go1.11 beta release, so I'm reverting it.

Fixes #25622

Change-Id: I758ade4af4abf764abd8336d404396992d11a0c6
Reviewed-on: https://go-review.googlesource.com/115535
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>