Use unittest2.TestCase.assertDictEqual.

The standard library version of unittest in CentOS 6 does not include an
assertDictEqual method. However, the third-party package unittest2 is a
backport of unittest features from later versions of Python.

This change modifies mlab_test to import unittest2 and use
assertDictEqual where appropriate.

Cleanup import order and third-party comment.
Remove unused traceback import.
Clarify fakes slice names to show site component.
Add successful test for read_vsys_data(_direct).

This change includes a successful test for read_vsys_data and supporting
method read_vsys_data_direct.
Add clarifying comment for virtual memory tests.
Clarify what values are timestamps.

Previously, the values used to simulate the passing of time were not
recognizable as timestamps. This patch tries to make this clearer.
Do not assert FakeVsysBackend shutdowns.
Simplify FakeVsysBackend shutdown handling.

This change removes the FakeVsysBackend methods complete_shutdown_reader
and complete_take_too_long. Instead, all event handling is moved to
shutdown. And, because both commands ('take_too_long' and
'shutdown_reader') are mutually exclusive, a single event is used for
both cases. Docstrings are updated accordingly.