fmt,math/big,net/url: fixes to old Benchmarks

fmt,math/big,net/url: fixes to old Benchmarks

b.ResetTimer used to also stop the timer, however it does not anymore.
These benchmarks hadn't been fixed and as a result ended up measuring
some additional things.

Also, make some for loops more conventional.

Change-Id: I76ca68456d85eec51722a80587e5b2c9f5d836a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/496996
Run-TryBot: Damien Neil <dneil@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>