The use of `let` is unnecessary here, particularly in a
file that is needed to run any of the tests, even the ES5 tests.
Removing it allows older engines to run older tests.

* Improve tests for BigInt.prototype.valueOf

* fixup! Improve tests for BigInt.prototype.valueOf

* fixup! Improve tests for BigInt.prototype.valueOf

* fixup! Improve tests for BigInt.prototype.valueOf

typeCoercion.js is included in several tests which are not marked as `features: [BigInt]`.
Since BigInt is new feature, we should not make the above unrelated tests failed due to
lack of BigInt syntax support.

Close #1252.

* BigInt valueOf tests

* add features from typeCoercion.js

* typeCoercion.js supports ToIndex
* typeCoercion.js supports ToBigInt
* updated BigInt.asIntN type coercion tests to use typeCoercion.js

After conversations in #1215, we need to catch up with the code that was previously
produced before we set the features flags requirements.

```
$ python tools/lint/lint.py --whitelist lint.whitelist harness/*.js
Linting 25 files.
Linting complete. 0 errors found.
```

- Renames detachArrayBuffer-$262.detachArrayBuffer.js -> detachArrayBuffer-host-detachArrayBuffer.js (previous naming prevents command-click from terminal)

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>