Add unit tests for setting preferred codec and using microphone

[DenisTensorWorks]

Component your feature request relates to

• Frontend library

Problem your feature solves? Please describe.
With the ability to set preferred codecs as well as add audio tracks to send back to the application, I propose adding unit tests for these features.

Describe the solution you'd like
These features can be tested by inspecting the offer/answer and searching for the suitable lines.

In the case of setting a preferred codec, we should expect this preferred codec to be the first in the SDP.

In the case of using the microphone, we should expect the audio track to be send/recv.

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Moved from EpicGames/PixelStreamingInfrastructure#197