fix & improve coverage of `estree` (#1935)

fix & improve coverage of `estree` (#1935)

- fix `estree` conversion of getter/setter
- fix non-directive literal in `to_mozilla_ast()`
- revamp `test/mozilla-ast.js`
  - reuse `test/ufuzz.js` for code generation
  - use `acorn.parse()` for creating `estree`
- extend `test/ufuzz.js` for `acorn` workaround
  - catch variable redefinition
  - non-trivial literal as directive
  - adjust options for tolerance

Miscellaneous
- optional semi-colon when parsing directives

fixes #1914
closes #1915