Write more tests against latest web2py `welcome` application

[matclab]

The objectives are twofold:

• having a better usage base and examples
• add some tests for the web2py plugin (which may perhaps be integrated in the default test suite in a more or less near future). It may particularly be of interest for the area not well tested on web2py (for example, testing mail interaction with authentication in web2py is not easy today, and is easier with the help of pytest-web2py and mocking).