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

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

@@ -0,0 +1,17 @@
+package p;
+
+enum SomeEnum {
+
+    FOO("input");
+
+    private String input;
+
+    private SomeEnum(String input) {
+        this.input = input;
+    }
+
+    public String getValue() {
+        return input;
+    }
+
+}