Fix a bunch of test TODOs.

- Added a bunch of tests.
- Removed some TODOs for stuff that's already done.
- Made list literals handle newlines better.

test/constructor/default.wren

@@ -2,10 +2,7 @@ class Foo {
   toString { return "Foo" }
 }
 
-// A class with no constructors gets an argument-less "new" one by default.
+// Classes inherit the argument-less "new" one by default.
 var foo = new Foo
 io.write(foo is Foo) // expect: true
 io.write(foo.toString) // expect: Foo
-
-// TODO: Test that class doesn't get default constructor if it has an explicit
-// one.