sweet/tile38: make the default benchmark shorter
sweet/tile38: make the default benchmark shorter

The purpose of such a high iteration count was to ensure the benchmark
reaches a steady-state with respect to the GC, but the count was way
overkill. This change reduces it.

Change-Id: I8e2f43e75e766a52347491ad37bb3835c167d717
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/378454
Trust: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Run-TryBot: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>

golang/benchmarks,golang/benchmarks