testing: address feedback for dev.fuzz merge

testing: address feedback for dev.fuzz merge

Based on comments in CL 348469.

Note that with this change, F.Fuzz no longer calls
runtime.Goexit. This simplifies our logic and makes F.Fuzz more
predictable.

Change-Id: I6c3c65b0e8e8f261621cbe2f17375e8164ef60a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/351316
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>