maps: move test funcs to maps_test.go

maps: move test funcs to maps_test.go

keysForBenchmarking and valuesForBenchmarking are only used in benchmark
tests.

Change-Id: Ie4fcb81e0470cc8627b395644787429b79952538
Reviewed-on: https://go-review.googlesource.com/c/go/+/497380
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Bypass: Ian Lance Taylor <iant@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>