Updated the Symbol test to also check Object.defineProperty

Based on a musing I had [here](https://github.com/kangax/compat-table/pull/163#issuecomment-49293344): this updates the Symbol test to check that:
* Symbols can be used with the `in` operator (`symbol in object`)
* Symbols can be used as keys in `Object.defineProperty` arguments, not just in bare assignments
This doesn't change any of the test's historical results, though.

This incidentally also alters the Symbol test and the `Object#__proto__` test to not rely on most ES5 `Object` static methods without checking for existence first (but `Object#__proto__` still requires `Object.getOwnPropertyDescriptor` for basic checking).