[COLLECTIONS-701] SetUniqueList.add() crashes due to infinite recursion.

Add tests.

src/test/java/org/apache/commons/collections4/list/Collections701Test.java

@@ -0,0 +1,42 @@
+package org.apache.commons.collections4.list;
+
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+
+import org.junit.Assert;
+import org.junit.Ignore;
+import org.junit.Test;
+
+/**
+ * Tests for COLLECTIONS-701.
+ */
+public class Collections701Test {
+
+    @Test
+    public void testArrayList() {
+        final List<Object> list = new ArrayList<>();
+        list.add(list);
+        Assert.assertEquals(1, list.size());
+        Assert.assertEquals(list, list.get(0));
+    }
+
+    @Test
+    public void testHashSet() {
+        final Set<Object> set = new HashSet<>();
+        set.add(set);
+        Assert.assertEquals(1, set.size());
+        Assert.assertEquals(set, set.iterator().next());
+    }
+
+    @Test
+    @Ignore
+    public void testSetUniqueList() {
+        final List<Object> source = new ArrayList<>();
+        final List<Object> list = SetUniqueList.setUniqueList(source);
+        list.add(list);
+        Assert.assertEquals(1, list.size());
+        Assert.assertEquals(list, list.get(0));
+    }
+}