Ensure assertions are evaluated in tests

A lot of assertions are used in Ganeti's code. Some unittests even check
whether AssertionError is raised in some cases. Explicitely ensuring
assertions are evaluated makes sure those tests don't fail and
assertions are checked.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>