Testing LibTracker::Client

So you downloaded LibTracker::Client and you even managed to compile it. But
all good modules have tests and so does LT::C. What makes LT::C a little
different is that it needs a running trackerd to test stuff. And it would
want to change some properties on an entry (path) indexed by tracker to test
out most of its functionality.

If LT::C were a regular old module, we could get around by using
Test::MockObject. But it isn't and we can't. Well, we _can_, but it would be
pointless. LT::C is essentially an XS wrapper around libtrackerclient, and
we would have to bypass the very XS-ish behaviour that we want to test, if
we go via the Test::MockObject route.

Another option is to mock the libtrackerclient library and build against
that, specifically for testing, but communicating with a real trackerd
instance is far easier and more real-world.

So - here's what the LT::C test interface assumes/expects :

$ENV{LTC_TRACKER_RUNNING} : if this is defined, LT::C will try connecting to
                            the trackerd instance.

$ENV{LTC_TEST_PATH}       : a path which is indexed by tracker. LT::C will
                            try to make (and revert) changes to this, but
                            naturally - tests might fail. It is good to keep
                            this as a test path. If this is not defined,
                            LT::C will skip all tests which require a path.

$ENV{LTC_META_FIELD}      : A string type metadata field that the test
                            scripts can play with. If this is not defined,
                            the test scripts assume 'File:Other'.