Missing coverage: BigInt.prototype.toString (partial)

[leobalter]

Missing coverage:

1. this value capturing objects with an [[BigIntData]] internal. (e.g.: BigInt.prototype.toString.call(Object(1n))).
2. ToInteger coercions and abrupt completions from radix.
3. Observable variants for the same this value using different radix, as long as the implementation defined spec permits.

[ryzokuken]

@leobalter please close this.