[mozsystemmonitor] Add test manifest to testing/mozbase/moz.build (bug 1471916) r=davehunt

Currently, tests in mozsystemmonitor are skipped because the manifest is missing
from mozbase/moz.build. This has now been added.

For the moment, this patch also skips the test if run against Python 3. This is
not ideal - a better fix may be setting the unittest.skipIf condition to skip
if the there is simply a failure when run with Python 3.

Differential Revision: https://phabricator.services.mozilla.com/D22426