Re-organize class tests

The `test/language/class/` directory contains a small subset of
Test262's tests for ES2015 classes. The majority of tests for classes
are organized within `test/language/statements/class/`.

- Move the tests that rely on the ClassDeclaration syntactic form from
  `test/language/class/` to `test/language/statements/class/`.
- Move the test that relies on the ClassExpression syntactic form from
  `test/language/class/` to `test/language/expressions/class/`.