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_static_method.wren

@@ -0,0 +1,10 @@
+class Foo {
+  static test {
+    IO.print(this == Foo) // expect: true
+    IO.print(this.bar) // expect: bar
+  }
+
+  static bar { return "bar" }
+}
+
+Foo.test