additional BigInt JSON tests

[cxielarko]

Additional JSON tests for BigInt, for JSON.stringify with a replacer function and the toJSON method.

[littledan]

These tests look valid to me. While you're at it, it'd also be good to have a few tests that fully verify the ordering,

1. toJSON
2. replacer
3. unwrap
4. TypeError if it's a BigInt

(this was a recent spec change)

[rwaldron]

Since there are updates to be made anyway (per @littledan's request), make all of these strings with single quotes so you can get rid of the escapes (both files)

[cxielarko]

This now includes an ordering test for JSON.stringify, and also uses single quotes to avoid string escaping in some places.