Add basic utils.muxer tests

Add basic utils.muxer tests

The tests in tests/plugins/test-utils-muxer.c create custom source and
sink components and connects them to a utils.muxer filter component
within a new graph.

The components do as follows:

* Source component: Depending on the current test, its notification
  iterators generate specific sequences of notifications.

* Sink component: For each notification, and some specific notification
  iterator statuses ("again", "end"), create a test event and append it
  to a global array of test events.

Each test creates a graph, runs it, and then compares the global array
of test events against a static list of expected test events.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>