Tests: python: enum.auto() introduced in python 3.6

Assign descriptive string values to the enumeration members instead of
using auto() and provide a repr() method that hides the value as
recommended upstream [1].

[1] https://docs.python.org/3.6/library/enum.html#omitting-values

Change-Id: I0bc8fcc19d68342ade1aeb587f07a9b483f81b3e
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>