encoding/json: move some misplaced benchmark tests to bench_test.go

encoding/json: move some misplaced benchmark tests to bench_test.go

Change-Id: I5987eed00ee825421abe62699a06e9b66499f35f
Reviewed-on: https://go-review.googlesource.com/c/go/+/425016
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>