stream: Add unit tests for channel stream usage.

This change adds unit tests cover the following:

1. That retrieving the first media stream of a specific media
type from a stream topology retrieves the expected media
stream.

2. That setting the native formats of a channel which does
not support streams results in the creation of streams on
its behalf according to the formats of the channel.

3. That setting a stream topology on a channel which supports
streams sets the topology to the provided one.

ASTERISK-26790

Change-Id: Ic53176dd3e4532e8c3e97d9e22f8a4b66a2bb755