@SafeVarargs
    @SuppressWarnings("varargs")
    private OrderTester<T> addGroup(T... group) {
        groups.add(List.of(group));
        return this;
    }
OrderTester is a an abstract helper class in the spirit of EqualsTester that
tests an objects total or partial ordering with respect to T#compareTo.

@author Vegard Sjonfjell
@see com.yahoo.test.TotalOrderTester
@see com.yahoo.test.PartialOrderTester
testutil/src/main/java/com/yahoo/test/OrderTester.java