“Dubin C.J.O. is frequently cited as succinctly articulating the test for determining a motion to intervene in Peel (Regional Municipality) v. Great Atlantic and Pacific Co. of Canada (1990), 1990 CanLII 6886 (ON CA), 74 O.R. (2d) 164: “[T]he matters to be considered are the nature of the case, the issues which arise and the likelihood of the applicant being able to make a useful contribution to the resolution of the appeal without causing injustice to the immediate parties.”