Fix Async-from-Sync Iterator tests

Add tests for the case of `<iterator>.return` in the iteration protocol being an object that's uncallable and compares equal to `undefined`

Add tests for the case of <iterator>.return, as used in the iteration protocol, being an object that's uncallable and compares equal to `undefined`.

CONTRIBUTING.md updates

This change updates test cases to assume that the "next" method is only
loaded from a synchronous iterator once, and is re-used for each call to
Async-from-Sync Iterator.next(), based on https://github.com/tc39/ecma262/pull/988

Make Atomics.wake test cases resilient on slow / overloaded systems

* BigInt.parseInt tests

* update for PR#1208

* fix copyright notices

Readme updates

ToPrimitive called without hint for boolean equality

BigInt arithmetic tests

Property descriptor tests for BigInt.name, BigInt.length, and BigInt.prototype.constructor

typeof test for BigInt

See relevant firefox bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1408452

The RegExp named groups specification has changed to not throw
errors in certain cases. This patch updates the test262 tests to match
the new specification, and throws in an additional test that verifies
the interaction between named group syntax and other replacement.
The tests pass on a version of V8 which implements the new semantics.

https://github.com/tc39/proposal-regexp-named-groups/commit/92ceba518c2ab0d2811c2efa8248ed1b3f8b5506

Updated tests for BigInt.prototype.toString

I'm leaving these todos for Leo or Rick -- I think you can explain the state and goal of Test262 better than me at this point!

* Delete identifier-let-allowed-as-lefthandside-expression-non-strict.js

Fixes #1192

This is already covered by https://github.com/tc39/test262/blob/8422147c17aa3f4061e524a8f88c251a3b371ecf/test/language/statements/for-in/head-lhs-let.js#L27

* restore for-in test

Add version of intl402/6.2.3.js which tests via Intl.getCanonicalLocales

Add messages to asserts and use mixed quotes instead of escapes