functional-tests: Improve helper code and share it with tracker-miners

This branch contains various style and organization improvements to the functional-tests. There shouldn't be any change in how they work.

The major change is that test helper code now lives in a separate trackertestutils Python package, which lives in the utils/ subdirectory of the source tree and will be installed into ${libdir}/tracker-2.0/trackertestutils. This change means we can share test helper code between tracker.git and tracker-miners.git, which will reduce maintenance effort for the tests.

Distributors should include this library as part of the tracker-devel package.

Another noticable change is that logging from the test helpers is now done using the Python 'logging' module, instead of using print() as before. This changes the output slightly when TRACKER_TESTS_VERBOSE is used. (In fact, we will soon be able to offer different levels of verbosity for the tests as we can now separate errors and warnings from debug info.)