Bluetooth: tester: Fix UART issue on qemu

This fixes an issue introduced by bab3aafa,
which changed the default value of UART_PIPE_ON_DEV_NAME to the same
used by BLUETOOTH_UART_ON_DEV_NAME. Due to this, the BTP and HCI were using
the same UART.

Change-Id: Ib59819e49610f28f4c89c0304d024bb1b6aa506a
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>