Adjust and rename Membrane.Testing.Pipeline to make it usable not only in tests

[mat-hek]

In the vast majority of scenarios, it's convenient to implement Membrane.Pipeline behaviour to have a pipeline. However, in some cases, it's overkill - for example in small demos, like the HLS demo. In such situations, we could do something like:

alias Membrane.Pipeline
{:ok, pipeline} = Pipeline.start_link()
Pipeline.exec_actions(spec: %ParentSpec{children: [hls_bin: ..., ...], ...})
Pipeline.subscribe(pipeline, :end_of_stream)
Pipeline.play(pipeline)
Pipeline.await(pipeline, :end_of_stream, :hls_bin)

The Membrane.Testing.Pipeline already has a similar API, so we could just rename fine-tune it. The new pipeline should be still usable in tests.