url_alter_test module uses an event subscriber instead of a path processor

The way that url_alter_test module alters the incoming path is wrong. It currently uses an event subscriber that listens to the request and sets the system path. This should have been changed with #1910318: Make path resolution the responsibility of a series of PathProcessor classes, rather than one PathSubscriber class but was inadvertently left out. The way to alter the incoming path is with a path processor. Patch attached...