StoreGate: Fix WriteDecorHandle::isAvailable.

WriteDecorHandle;:isAvailable had a couple problems. First, the isAvailable code assumed that the object type derived from AuxVectorBase, which means that it didn't work at all for standalone types. Second, calling isAvailable would populate the cached pointer without calling the overload of typeless_dataPointer_impl. This meant that the alias might not be created.

Both fixd.

Also clean up the tests so that we don't need to override the JO search path to run them.