Bluetooth: Host: Shell: Add missing guard for CS test commands

The start_simple_cs_test and stop_cs_test depends on
CONFIG_BT_CHANNEL_SOUNDING_TEST and could cause build errors if that
is disabled while CONFIG_BT_CHANNEL_SOUNDING=y in the shell.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>