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 <[email protected]>

Author: sjanc

tests/bluetooth/tester/prj.conf

@@ -12,7 +12,7 @@ CONFIG_BT_SMP_ALLOW_UNAUTH_OVERWRITE=y
 CONFIG_BT_SMP_APP_PAIRING_ACCEPT=y
 CONFIG_BT_SIGNING=y
 CONFIG_BT_BONDABLE=y
-CONFIG_BT_ATT_PREPARE_COUNT=6
+CONFIG_BT_ATT_PREPARE_COUNT=8
 CONFIG_BT_GATT_CLIENT=y
 CONFIG_BT_L2CAP_DYNAMIC_CHANNEL=y
 CONFIG_BT_DEVICE_NAME="Tester"