Poor test coverage of Object.isExtensible with non-object argument

[linusg]

There are tests for this but they only check that the function doesn't throw, not whether it returns true or false:

• https://github.com/tc39/test262/blob/8b30689/test/built-ins/Object/isExtensible/15.2.3.13-1-1.js
• https://github.com/tc39/test262/blob/8b30689/test/built-ins/Object/isExtensible/15.2.3.13-1-2.js
• https://github.com/tc39/test262/blob/8b30689/test/built-ins/Object/isExtensible/15.2.3.13-1-3.js
• https://github.com/tc39/test262/blob/8b30689/test/built-ins/Object/isExtensible/15.2.3.13-1-4.js

I had a logic error in Kiesel's implementation of Object.isExtensible since day one that went unnoticed until now (likely copy/paste bug from Object.isFrozen), thanks to this test which should not pass if Promise.allKeyed is undefined:

• https://github.com/tc39/test262/blob/8b30689/test/built-ins/Promise/allKeyed/extensible.js