Factor out minimal test to be used as base for custom test

This will allow to start the building of the tests for an application
based on Eve by subclassing the TestMinimal class.
Remove few assert: they are in the TestMinimal class already
Add docstring to TestMinimal
Merge branch 'feature/test-minimal' of https://github.com/ahref/eve into ahref-feature/test-minimal
TestMinimal class added to the test suite
Daniele Pizzolli
Merge branch 'ahref-feature/test-minimal' into develop