Previously, ToddlerBase and derived classes were used directly, but none
of the methods is marked as a class or static method. Amazingly enough,
it worked regardless. It's useful to have a constructor, though, so do
things conventionally.
As we can't just monkey-patch the classes then, use the shared `toddler`
fixture which creates the object for the test methods that need it.
Signed-off-by: Nils Philippsen <nils@redhat.com>
