tty: n_smux: Update subsystem restart unit tests

For subsystem restart unit tests, add check to make sure the remote
system is back up before continuing with the tests.  This allows
removing fixed wait times in the unit tests and allows stress testing
using a shell script to call tests back-to-back.

Change-Id: I9af2164dd2ff8bc4d8a1e57f8a7aad957528a8c9
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>