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

java/ql/test/utils/modelgenerator/dataflow/CaptureNeutralModels.expected

@@ -3,6 +3,7 @@
 | p;FluentAPI$Inner;notThis;(String);summary;df-generated |
 | p;ImmutablePojo;getX;();summary;df-generated |
 | p;Joiner;length;();summary;df-generated |
+| p;MultipleImpl2$IInterface;m;(Object);summary;df-generated |
 | p;ParamFlow;ignorePrimitiveReturnValue;(String);summary;df-generated |
 | p;ParamFlow;mapType;(Class);summary;df-generated |
 | p;Pojo;doNotSetValue;(String);summary;df-generated |

java/ql/test/utils/modelgenerator/dataflow/CaptureSummaryModels.expected

@@ -22,6 +22,7 @@
 | p;Joiner;false;setEmptyValue;(CharSequence);;Argument[0];Argument[this];taint;df-generated |
 | p;Joiner;false;setEmptyValue;(CharSequence);;Argument[this];ReturnValue;value;df-generated |
 | p;Joiner;false;toString;();;Argument[this];ReturnValue;taint;df-generated |
+| p;MultipleImpl2$IInterface;true;m;(Object);;Argument[0];ReturnValue;taint;df-generated |
 | p;MultipleImpls$Strat2;true;getValue;();;Argument[this];ReturnValue;taint;df-generated |
 | p;MultipleImpls$Strategy;true;doSomething;(String);;Argument[0];Argument[this];taint;df-generated |
 | p;MultipleImpls$Strategy;true;doSomething;(String);;Argument[0];ReturnValue;taint;df-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;
+        }
+    }
+}