bug 1401246: Convert adapter.clean_username tests
bug 1401246: Convert adapter.clean_username tests

Django 1.9 updates assertRaisesMessage so that the test becomes:

<message> in str(<exception>)

where our <message> is a lazy-evaluated translated string, and results
in a TypeError:

'in <string>' requires string as left operand, not __proxy__

This fixes the TypeError by converting it to a regular string, such as
str(USERNAME_EMAIL), and also converts the tests to pytest style,
entirely avoiding assertRaisesMessage magic.

Elchi3/kuma,a2sheppy/kuma,mozilla/kuma,a2sheppy/kuma,jwhitlock/kuma,SphinxKnight/kuma,SphinxKnight/kuma,mozilla/kuma,SphinxKnight/kuma,a2sheppy/kuma,SphinxKnight/kuma,mozilla/kuma,SphinxKnight/kuma,escattone/kuma,jwhitlock/kuma,jwhitlock/kuma,Elchi3/kuma,mozilla/kuma,jwhitlock/kuma,escattone/kuma,Elchi3/kuma,a2sheppy/kuma,Elchi3/kuma,a2sheppy/kuma,SphinxKnight/kuma,escattone/kuma,jwhitlock/kuma,Elchi3/kuma,mozilla/kuma