py/objlist: Make list += accept all arguments and add test.

tests/basics/list_extend.py

@@ -0,0 +1,33 @@
+# test list.__iadd__ and list.extend (they are equivalent)
+
+l = [1, 2]
+l.extend([])
+print(l)
+
+l.extend([3])
+print(l)
+
+l.extend([4, 5])
+print(l)
+
+l.extend(range(6, 10))
+print(l)
+
+l.extend("abc")
+print(l)
+
+l = [1, 2]
+l += []
+print(l)
+
+l += [3]
+print(l)
+
+l += [4, 5]
+print(l)
+
+l += range(6, 10)
+print(l)
+
+l += "abc"
+print(l)