Improve `for..of` control flow tests

- Expand ambiguous assertion messages and assert execution paths more
  finely.
- Improve variable names in `for..of` tests
  While the object created by a GeneratorFunction may be considered an
  "iterable", it is being used as an iterator in these tests. Naming the
  variable according to the way it is used improves the readability of
  the test body.
- Add 'features' attribute to test frontmatter
- Move tests
- Introduce additional `for..of` control flow tests