Raise diskfile.py module coverage to > 98%

We attempt to get the code coverage (with branch coverage) to 100%,
but fall short because due to interactions between coverage.py and
CPython's peephole optimizer. See:

    https://bitbucket.org/ned/coveragepy/issue/198/continue-marked-as-not-covered

In the main diskfile module, we remove the check for a valid
"self._tmppath" since it is only one of a number of fields that could
be verified and it was not worth trying to get coverage for it. We
also remove the try / except around the close() method call in the
DiskFileReader's app_iter_ranges() method since it will never be
called in a context that will raise a quarantine exception (by
definition ranges can't generate a quarantine event).

We also:

* fix where quarantine messages are checked to ensure the
  generator is actually executed before the check
* in new and modified tests:
  * use assertTrue in place of assert_
  * use assertEqual in place of assertEquals
* fix references to the reserved word "object"

Change-Id: I6379be04adfc5012cb0b91748fb3ba3f11200b48
Merge "Raise diskfile.py module coverage to > 98%"