encoding/japanese: no need for a "package japanese_test".

The other ../*/all_test.go files are all "package foo", not "package foo_test".

Change-Id: Ic2a3f12c43b68cecff4afa3b3c9513e4a4e8e2dc
Reviewed-on: https://go-review.googlesource.com/37411
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>