The test data was modified so that support for catalog only mode
of kiwix-serve can be properly tested.
The effect of this change in the test data on the library_server unit
test demonstrates that although the new entry does not appear in the
catalog (for example, no catalog_v2_entries* test cases were affected)
the category and language of this ghost entry slipped into the
observable output.
We want to be sure that `Library` actually exists when we modify it.
While it is not a silver bullet (user can still create a shared_ptr on
a raw pointer), making the `Manager` keep `shared_ptr` on the library
help us a lot here.
