testing: fix Benchmark() to start at 1 iteration, not 100

testing: fix Benchmark() to start at 1 iteration, not 100

The run1 call removed in golang.org/cl/36990 was necessary to
initialize the duration of the benchmark. With it gone, the math in
launch() starts from 100. This doesn't work out well for second-long
benchmark methods. Put it back.

Updates #18815

Change-Id: I461f3466c805d0c61124a2974662f7ad45335794
Reviewed-on: https://go-review.googlesource.com/37530
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>