reflect: add a copy of BenchmarkMap

This is a copy of reflect_test.BenchmakMap from the Go standard library,
added in http://golang.org/cl/345486.

This is added to x/benchmarks only temporarily as a sample 'go test'
benchmark to use with benchmarking tools. This benchmark was selected in
particular because it had a significant performance improvement with a
recent upstream change (http://golang.org/cl/345486).

Change-Id: I2bff47ffbaab1d471a37ec3cba4bea88cc78d8ed
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/353649
Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>