feat(material-experimental/mdc-snack-bar): add test harness #20366
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Adds a test harness for the MDC-based snack bar. One gotcha here compared to the standard snack bar is that the harness excludes (via a CSS selector) instances that are in the process of being closed. We have to take this approach, because MDC's animations are run outside of Angular and we don't have a way of waiting for them to finish.