Bug 1552914 - [mozlog] Update formatters to reflect new known_intermittent field. r=jgraham

The in-tree log formatters have been updated to reflect the new known_intermittent
field, ensuring that a status matching a known_intermittent status is not logged as an
unexpected failure. A message is printed when there is a test status that matches this.
A test for known intermittents has been added to the test_formatters, following the
same testing style for pass or fail.

Differential Revision: https://phabricator.services.mozilla.com/D32174