Add another test for constructor syntax.

test/constructor/infix_class_expression.wren

@@ -0,0 +1,7 @@
+class Foo {
+  + other { return "Foo " + other }
+}
+
+io.write(new Foo + "value") // expect: Foo value
+
+// TODO: Other expressions following a constructor, like new Foo.bar("arg").