When writing tests, for performance reasons we often want to reuse
the same data-source instance for many tests suites. At the same time,
we want to keep such test suites independent and allow them to reuse
the same model name for different model classes.
Juggler does support redefinition of a model with the same name.
This change is adding a new API called that allows tests to remove all
old models before creating new ones. This API would be typically
called from a `before` hook.
Signed-off-by: Miroslav Bajtoš <mbajtoss@gmail.com>