Add unit test verifying that java_object_factory doesn't strip tag types

Previously if asked to populate a pointer with tagged type (e.g. "Object *") then it would
allocate a "struct Object { ... }", i.e. the struct_typet rather than the struct_tag_typet.
This verifies that it now retains tag types.

Author: smowton

jbmc/unit/java_bytecode/java_object_factory/A.class

@@ -0,0 +1,11 @@
+
+public class A {
+
+  public B b;
+  public C c;
+
+}
+
+class B { }
+
+class C { }

jbmc/unit/java_bytecode/java_object_factory/A.java

@@ -0,0 +1,8 @@
+
+public class HasArray {
+
+  public D[] ds;
+
+}
+
+class D { }