* Handle det == 0 in findCircle3pts.

Issue 16051 shows a case where findCircle3pts returns NaN for the
center coordinates and radius due to dividing by a determinant of 0. In
this case, the points are colinear, so the longest distance between any
2 points is the diameter of the minimum enclosing circle.

* imgproc(test): update test checks for minEnclosingCircle()

* imgproc: fix handling of special cases in minEnclosingCircle()