test: add test verifying this refers to class in static methods

Add a new test file `this_in_static_method.wren` that validates `this` evaluates to the class object when used inside a static method, and removes the TODO comment from `this_in_method.wren` that previously noted this untested behavior.

test/this/this_in_method.wren

@@ -4,7 +4,3 @@ class Foo {
 }
 
 IO.print((new Foo).bar.baz) // expect: baz
-
-// TODO: Test that "this" is the class when in a static method.
-//       (Or disallow "this" in statics? It's useful since statics are
-//       inherited.)