This exercises the multi-threaded codepath and also tests precision
a bit higher.
To avoid quadratic blowup, only a short set of formats are tested in
the larger conversion tests.
Task-number: QTBUG-82818
Change-Id: I411deb97aea61a69fbdb24cbaf6559dd9436b703
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>