sandbox: Use separate dirs for config, data, etc.

It's not a good idea to set XDG_CONFIG_HOME, XDG_DATA_HOME, etc. to
the same directory, because some software might store a file
with the same name in two of those directories. DConf does it.
functional-tests: Set up data directores like tracker-sandbox does

To debug a functional test failure you can now replace the 'rm' at the
bottom of 'test-runner.sh' with 'echo', and then call:

    tracker-sandbox.py -i /tmp/tracker-test-XXXX/tracker-tests --shell