tests: bluetooth: tester: Increase ATT prepare write buffers count
tests: bluetooth: tester: Increase ATT prepare write buffers count

GATT/SR/GAW/BV-10-C requires more buffers as it tests nested long
writes.

Signed-off-by: Szymon Janc <db186c297188f7acbbdc9bbd6aff2f45665bb42d@codecoup.pl>

zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr