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

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Asserts that pipeline received or will receive a message matching message_pattern from another process within the timeout period specified in milliseconds.

The message_pattern must be a match pattern.

assert_pipeline_receive(pid, :tick)

Such call would flunk if the message :tick has not been handled by c:Membrane.Parent.handle_other/3 within the timeout.