Add tests for Unicode escape sequences

- Re-organize existing tests for identifiers
  Name files according to their content (not their ES5 ID). Move tests
  for IdentifierName and LabelIdentifier to more appropriate directories.
- Simplify and correct tests
  Instead of asserting successful runtime evaluation using `eval`, rely
  on the test runner's ability to detect syntax errors. Update the test
  bodies to test the grammar referenced by their ES5 ID and
  description--the IdentifierStart pattern.
- Use `negative` frontmatter to assert SyntaxErrors
- Remove redundant tests
- Use `assert.equal` helper function
- Add equivalent tests for literal unicode chars
- Add tests for variable-length unicode escape