Fix issues that appeared since the tracker-writeback was fixed
The tracker-writeback daemon was broken for a long time. It was recently fixed by tracker!96 (merged).
This merge request fixes some follow up issues:
- the 'enable-writeback' config option for tracker-miner-fs didn't work as expected, now it does
- the functional-410-extractor-decorator test started failing, now it passes again
- the writeback functional tests were skipped, now they are enabled and passing
- the functional tests could spawn the tracker-writeback (or any other tracker daemon) from /usr; now they can only spawn the daemons in the build tree