We already have a libmpv test, but it's really basic and barely does
anything. In an effort to hopefully prevent future breakage, lets expand
this to do more stuff. First of all, it simply tests setting a bunch of
properties/options. Secondly, it tries to load a file (the mpv icon that
we already have in the source tree). And then finally it tries the
lavfi-complex filter since this is a known special case where weird
things can happen.