Extend coverage for RegExp.prototype[Symbol.replace]

[leobalter]

Based on findings from #2423:

1. Extend tests coverage for RegExp.prototype[Symbol.replace]
2. Create a feature flag for RegExp.prototype[Symbol.replace] and String.prototype.replace. Not new features, but there are too much tangliness here. Yes, I invented a new word.
3. Extend tests coverage for String.prototype.replace
4. Remove redundant legacy tests for String#replace we will eventually find as duplicates of the new coverage from the previous item.

Originally posted by @leobalter in #2423