Add test for default constructor.

test/constructor/default.wren

@@ -0,0 +1,11 @@
+class Foo {
+  toString { return "Foo" }
+}
+
+// A class with no constructors gets an argument-less "new" one by default.
+var foo = Foo.new
+io.write(foo is Foo) // expect: true
+io.write(foo.toString) // expect: Foo
+
+// TODO(bob): Test that class doesn't get default constructor if it has an
+// explicit one.