Improve tracker-sandbox for tests
This fixes some issues, and adds a new --index-recursive-tmpdir
flag which causes the tool to create a tmpdir and configure it for indexing by Tracker.
The idea is that packages can run test programs with tracker-sandbox --index-recursive-tmpdir -- my-test
and using the TRACKER_INDEXED_TMPDIR
env var they can create content that the miners will index, without needing to faff with GSettings configuration in the test itself.