testing: implement -cpu and -count for fuzz tests

testing: implement -cpu and -count for fuzz tests

Fuzz tests are meant to be run just like ordinary tests,
so copy the same loop cpu and count loops used in testing.go
(and benchmark.go) into fuzz.go.

Change-Id: Ic585df8ccc577869c877b1055e0493803dbeb828
Reviewed-on: https://go-review.googlesource.com/c/go/+/443377
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>