cmd/bench: run 10 iterations for each bent benchmark

For golang/go#49207.

Change-Id: I83fa87a603cf26ed61d324975166388db1801487
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/382097
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>