syncmap: add benchmark for Range

adjust BenchmarkAdversarialDelete to be somewhat more adversarial.

updates golang/go#18177

Change-Id: Id01ed1077a0447dcfc6ea3929c22baaddbc9d6ee
Reviewed-on: https://go-review.googlesource.com/37151
Reviewed-by: Russ Cox <rsc@golang.org>