Usability testing and contextual interview are very similar methods. Many specialists just have a tendency to focus on narrow, strictly-beforehand-defined questions in usability tests. This approach does not leave much space for learning about user activities, real experiences and real situations of use. By contrast in contextual interviews, questions and tasks are typically open-ended and related to real user activities. The line between usability test and contextual interview is blurred, like the difference between structured and thematic interviews.
Design people tend to have a negative attitude towards usability testing and research. This is probably due to a narrow understanding of usability and the strategy of easy wins, both strongly driven by usability people like Jakob Nielsen. In a wider sense, usability means how appropriate, practical and pleasant services and products are to their real users in real situations. This is the “real” meaning of usability, defined in the ISO standard #9241. Usability is much more than big fonts and contrasts when it is developed in an appropriate, contextual way.