You're seeing just the macro assert_end_of_stream, go back to Membrane.Testing.Assertions module for more information.
Link to this macro

assert_end_of_stream(pipeline, element_name, pad \\ :input, timeout \\ 2000)

View Source (macro)

Asserts that Membrane.Testing.Pipeline received or is going to receive end_of_stream notification about from the element with a name element_name within the timeout period specified in milliseconds.

assert_end_of_stream(pipeline, :an_element)