This is the equivalent of #127 (and #140) for CardDAV and finally allows
backends to serve different address books to different users.
While I'm breaking the interface, correct one last instance of
"Addressbook" to "AddressBook" (in `AddressBookHomeSetPath`).
As the implementation evolves, it will be necessary to have more tests
to assert we don't break anything when making changes. This commit
introduces a test setup to test that server and client can handle the
address book discovery with various parameters. The test setup should be
easily extendable to cover even more ground as needed.
