Get rid of ListMultimap usage in SelectorTestUtils if reasonable

The less we depend on guava, the better - due to their backwards compatibility policy of 18 months.
On the other hand, we don't want to copy paste code into optaplanner just to avoid that dependency (some of our upstream depend on it anyway).

In SelectorTestUtil.java, we're using:
import com.google.common.collect.ImmutableListMultimap;
import com.google.common.collect.ListMultimap;

but I think that now that we use Java 8, there's probably a better way than having to resort to a ListMultimap.