Remove few PendingDeprecationWarning from tests

when building in Buster.

============================= test session starts ==============================
platform linux2 -- Python 2.7.16rc1, pytest-3.10.1, py-1.7.0, pluggy-0.8.0 -- /usr/bin/python2
cachedir: .pytest_cache
rootdir: /srv/dak, inifile:
collecting ... collected 166 items

[...]

=============================== warnings summary ===============================
tests/test_parse_changes.py::ParseChangesTestCase::test_3
  /srv/dak/tests/test_parse_changes.py:95: PendingDeprecationWarning: Please use assertFalse instead.
    self.failIf(changes.get('you'))
  /srv/dak/tests/test_parse_changes.py:95: PendingDeprecationWarning: Please use assertFalse instead.
    self.failIf(changes.get('you'))
  /srv/dak/tests/test_parse_changes.py:95: PendingDeprecationWarning: Please use assertFalse instead.
    self.failIf(changes.get('you'))

tests/test_parse_changes.py::ParseDscTestCase::test_9
  /srv/dak/tests/test_parse_changes.py:66: PendingDeprecationWarning: Please use assertFalse instead.
    self.failIf(changes.get('this'))

-- Docs: https://docs.pytest.org/en/latest/warnings.html
=================== 166 passed, 4 warnings in 19.14 seconds ====================
3 jobs for remove-pending-deprecation-warnings in 5 minutes and 43 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #147200
flake8

00:01:51

passed #147199
test buster

00:05:43

passed #147198
test stretch

00:05:06