The testoon was the anglicised version of the Italian testone. A quick wiki check suggests the testone ceased to be struck in the1550s. The nordic skilling wasn't introduced until 1625, so looks to have been adopted from the English terminology. The scilling was an ancient accounting term, just as the merk was, and is likely to be the origin of the value's useage. As for the changeover in terminology, mint records covering the commission to John Bowes at Durham House of 29th Jan 1548/9 refer to the shilling and half-shilling, and the conversion of testoons (i.e. the debased HenryVIII issues). It seems likely that the change was initially aimed at distinguishing between the old and new coins. Subsequently, there is reference to shillings of varying fineness, so the use of testoon was restricted to the issues they were trying to call in and melt down, and is referred to as such when the maximimum levy for exchanging the old money was set at 2d in every 20s.