Add another unit test for operator overloading

unittests/operator_overload2.chai

@@ -0,0 +1,8 @@
+def Bob::Bob() { }
+attr Bob::x
+def `-`(a, b) : is_type(a, "Bob") && is_type(b, "Bob") { a.x - b.x }
+var b = Bob()
+var c = Bob()
+b.x = 4
+c.x = 5
+print(b-c)

unittests/operator_overload2.txt

@@ -0,0 +1 @@
+-1