Add escape-above-astral.js to cover escaping code units as encoded from extended unicode escape. (#942)

Fix 850

Fixes #887

test/annexB/built-ins/Date/prototype/setYear/time-clip.js
test/built-ins/Date/prototype/setFullYear/new-value-time-clip.js
test/built-ins/Date/prototype/setMonth/new-value-time-clip.js
- Don't try to test time-clip at the end points, because this is near
impossible to get right (needs to consider time zone offset, dst, local
mean time because of Africa/Monrovia, etc.).

test/built-ins/DataView/prototype/setFloat64/detached-buffer-after-toindex-byteoffset.js
test/built-ins/DataView/prototype/setInt16/detached-buffer-after-toindex-byteoffset.js
- Wasn't update to expect RangeError

test/built-ins/Function/internals/Construct/derived-this-uninitialized-realm.js
- Change ClassDeclaration -> ClassExpression to get completion value

test/built-ins/Function/prototype/toString/AsyncFunction.js
- Add missing \n in expected string
- Also fixed in gh-847

test/built-ins/global/global-object.js
- Add 'var' to make test pass in strict-mode

test/language/block-scope/syntax/redeclaration-in-block/attempt-to-redeclare-function-declaration-with-function-declaration.js
- This is allowed in sloppy mode when Annex B is implemented

test/language/expressions/async-generators/expression-yield-as-statement.js
- Fix calls to then()

test/language/module-code/namespace/internals/own-property-keys-binding-types.js
test/language/module-code/namespace/internals/own-property-keys-sort.js
- Tests weren't updated after removal of @@iterator from module
namespace objects

test/language/module-code/namespace/internals/set-prototype-of-null.js
- Fix syntax error

test/language/statements/async-function/early-errors-no-async-generator.js
- No longer valid now that async iteration proposal is at stage 3

- Add tests for abrupt completion handling from ToString operations
- Add tests for method property descriptors
- Remove redundant assertion

Extend tests for the `escape` and `unescape` built-ins to assert their
behavior. Rely on the latest ECMAScript draft, which specifies that
`escape` returns hexadecimal codes in upper case.

Introduce a test that asserts the equality of
`Date.prototype.toGMTString` and `Date.prototype.toUTCString`, rendering
further tests for the former's behavior unnecessary.

This harness function is not necessary in the majority of cases in which
it is used. Remove its usage to simplify tests and decrease the amount
of domain-specific knowledge necessary to contribute to the test suite.

Persist the harness function itself for use by future tests for ES2015
modules (such a helper is necessary for tests that are interpreted as
module code).

The "mainline" tests in Test262 are converging on a more formal
structure. Files are organized as tests for either either "language"
(e.g. syntax-driven) or "built-in" (e.g. API-driven). "Language" test
locations are themselves structured according to whether the syntactic
form under test is an Expression or a Statement.

To limit ambiguity when locating/adding tests, re-organize the tests for
Annex B extensions to match this structure.