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

[linusg]

Same as #4981, e.g.:

• https://github.com/tc39/test262/blob/8b30689/test/built-ins/Object/seal/seal-null.js
• https://github.com/tc39/test262/blob/8b30689/test/built-ins/Object/seal/seal-undefined.js
• https://github.com/tc39/test262/blob/8b30689/test/built-ins/Object/seal/seal-symbol.js