Updated filterjournald for pyjournalctl 0.5.0

Now handles rare case of multiple of the same field in log entry.
Also no longer has issue with GIL on blocking calls
Added tests for journald backend

Due to actually having to use the journal for testing, the test cannot
use the modified TZ and time environment as the other tests do. Therefore
the tests for journald backend run in separate test at end of other
tests (if applicable).