avocado.utils.params: Add code to access test params

Access test params through its internal dictionary, so
you can lookup

self.params.timeout

As well as

self.params['timeout']

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
tests: Add multiplextest

Add a test that exercises the multiplex system, as well
as the system to access parameters passed to the test
(self.params).

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>