add test for perimeter

TriangleTest.java

    @Test
    void trianglePerimeterTest() {
        // Copyright (C) Marco Marinello <[email protected]>
        assertTrue(new Triangle(new Point(6,4), new Point(14,4), new Point(6,10)).perimeter() == 24);
        assertTrue(new Triangle(new Point(6,10), new Point(6,4), new Point(14,4)).perimeter() == 24);
        assertTrue(new Triangle(new Point(14,4), new Point(6,10), new Point(6,4)).perimeter() == 24);

        assertTrue(new Triangle(new Point(30,15), new Point(45,35), new Point(60,15)).perimeter() == 80);
    }

nice one 👍