tty: n_smux: Add wakeup test case

Reproducing and isolating wakeup issues caused by incorrect UART
configuration and hardware issues currently requires analysis of the
internal logs which is often time consuming.

Add wakeup test case to allow teams to easily reproduce wakeup issues or
verify wakeup functionality.

Change-Id: I614d6ee3eefc6563930c45564b76452e89f65dc8
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>