tests: Hide INFO and DEBUG messages by default
Tracker's DEBUG and INFO messages are very verbose -- thousands of lines each time you open a TrackerSparqlConnection, which some tests do many times. Let's hide the output by default, guarded behind the TRACKER_TESTS_VERBOSE environment variable which the functional-tests are already using.
The size of the meson-logs.txt file reduces from 27MB to about 100KB with this change.
Any really useful debug messages which are being lost after this change could be promoted to g_message() to ensure they are always logged.