tests: bluetooth: MESH shell is observer not central
tests: bluetooth: MESH shell is observer not central

Use BT_OBSERVER instead of BT_CENTRAL for this test.

Signed-off-by: Pawel Dunaj <ea2ed61998e18f01fdc731077c63bf3b1fed950c@nordicsemi.no>

zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,ldts/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,ldts/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,ldts/zephyr,ldts/zephyr