serial_io.py: Update Unittests

The existing unit tests were not working correctly after #60. They were now
reviewed and should work okay.

See GitHub Issue #70 for details.

Signed-off-by: Erik Bernoth <erik.bernoth@gmail.com>
Acked-by: Steffen Sledz <sledz@dresearch-fe.de>
Merge branch 'b-serialio-forgotten-testing' into dev

Finishes GitHub Issue #70.