add "assert.js" to python, website runners too

Validated fix for python runner ---

```
$ ./tools/packaging/test262.py --command='node --harmony' prototype.find_this-defined
es6/Array.prototype.find/Array.prototype.find_this-defined passed in non-strict mode
```

Theoretical (unvalidated) fix for website runner.

We need some sort of test framework for this stuff -- and I know it sounds nuts to have a test framework for a test framework, but there it is.