games/kanatest: fix ports compliance

• change += into = where appropriate

While the Porter's Handbook wants us to place japanese as the first
category, Poudriere wants games to be the first category. Given that
an unhappy Poudriere leads to a broken port, this was not changed.

Reported by: eduardo
Approved by: portmgr (blanket)
Approved by: eduardo (mentor)
Differential Revision: https://reviews.freebsd.org/D38632