tests: test case for missing after_configured signal

[lnielsen]

These lines needs a test case.

Description of bug:

• sa.orm.configure_mappers() sends the after_configured signal only if there are new model classes
• SQLAlchemy-Continuum relies on after_configured signal to call configure_versioned_classes() which builds the model version classes.
• If all models have been loaded and sa.orm.configure_mappers() has been called prior to SQLAlchemy-Continuum being initialised, then the versioned classes will not be created without this extra call