Not sure about the test program but during my time at RAF Shawbury, instructing on the Gazelle, one of our squadron tasks was teaching future Harrier pilots to hover.
I wasn't too keen on doing this because the hover height (50 feet agl, iirc), put us in the avoid curve for the aircraft.
Having been lucky enough to fly the RAFG twin seat Harrier (just the once, unfortunately), I found how easy it was to hover, very stable.