You're seeing just the macro assert_pipeline_playback_changed, go back to Membrane.Testing.Assertions module for more information.
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assert_pipeline_playback_changed(pipeline, previous_state, current_state, timeout \\ 2000)

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Asserts that pipeline's playback state (see Membrane.PlaybackState) changed or will change from previous_state to current_state within the timeout period specified in milliseconds.

The prev_state and current_state must be match patterns.

Assertion can be either made by providing state before and after the change as atoms:

   assert_pipeline_playback_changed(pipeline, :prepared, :playing)

Or by using an _ to assert on change from any state to other:

  assert_pipeline_playback_changed(pipeline, _, :playing)