runtime: more thorough map benchmarks

runtime: more thorough map benchmarks

Based on the benchmarks in github.com/cockroachlabs/swiss.

For #54766.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest-swissmap
Change-Id: I9ad925d3272c671e21ec04eb2da5ebd8f0fc6a28
Reviewed-on: https://go-review.googlesource.com/c/go/+/596295
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>