Fix errors in tracker-miner-fs test

Since cef502e6 we have been violating one of the constraints of TrackerTaskPool, which is that there should be only one task per GFile in the pool. On item_move() events, tracker-miner-fs would queue one task to remove the old destination file, then a second task to update the URL of the new destination file. These would conflict with each other as they were associated with the same GFile, leading to "Parent 'file:///tmp/tracker-miner-fs-test-77E2LZ/recursive/4' not indexed yet" errors and possibly other problems.

This branch fixes the issue, and adds a g_warning() on tracker_task_pool_add() to guard against future regressions.

It also fixes a small issue in the tracker-miner-fs test which led to two of the test cases never being run.

Fixes #15 (closed)