Duration query would return TRUE and duration=-1. This
worked in the unit test because the unit test implementation
was a bit broken.

Both queries need to access rate with a lock.

Fix broken duration query test as well. It relied on broken
behaviour by the videorate query handler, and also it was
implemented as a downstream query rather than an upstream
query. And we must return HANDLED from the probe so that the
query we intercept actually returns TRUE.

https://bugzilla.gnome.org/show_bug.cgi?id=699077