Java: Move dataflow based model generation testcases to a subfolder.

java/ql/test/utils/model-generator/dataflow/p/FluentAPI.java

@@ -0,0 +1,15 @@
+package p;
+
+public final class FluentAPI {
+
+    public FluentAPI returnsThis(String input) {
+        return this;
+    }
+
+    public class Inner {
+        public FluentAPI notThis(String input) {
+            return FluentAPI.this;
+        }
+    }
+
+}