Java: Add a testcase where both a neutral summary and summary is being generated.

java/ql/test/utils/modelgenerator/dataflow/p/MultipleImpl2.java

@@ -0,0 +1,20 @@
+package p;
+
+class MultipleImpl2 {
+
+    public interface IInterface {
+        Object m(Object value);
+    }
+
+    public class Impl1 implements IInterface {
+        public Object m(Object value) {
+            return null;
+        }
+    }
+
+    public class Impl2 implements IInterface {
+        public Object m(Object value) {
+            return value;
+        }
+    }
+}