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

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Asserts that Membrane.Testing.Sink with name sink_name has not received and will not receive caps matching caps_pattern within the timeout period specified in milliseconds.

Similarly as in the assert_sink_caps/4 the_sink needs to be part of a Membrane.Testing.Pipeline.

refute_sink_caps(pipeline, :the_sink, %Caps{prop: ^val})

Such expression will flunk if the_sink received or will receive caps with property equal to value of val variable.