Make tools/generate-test-fixture.js output "proper" JSON by quoting obje...

...ct keys

Currently tools/generate-test-fixture.js outputs JSON objects whose keys are not
double-quoted. This changes the tool to always double-quote object keys thus
resulting in the ability to pipe the output of generate-test-fixture.js to other
tools that expect valie JSON.

https://code.google.com/p/esprima/issues/detail?id=461