Some of these are based on work by André Bargull in
<https://bugzilla.mozilla.org/show_bug.cgi?id=1433303>.

- "CannotSuspendMainAgent" feature was changed to "CanBlockIsFalse" flag
- Move annex-b tests into annex-b directory
- Update variable names in nonshared-int-views.js tests
- Move getReport() call in nan-for-timeout.js to avoid iloop
- Update BigInt constructor to match new semantics (tc39/proposal-bigint#138)

Fixes #1492

Fleshed out tests for Symbol.matchAll, String.p.matchAll, RegExp.p[@@matchAll], and %RegExpStringIteratorPrototype%

Tests were updated and assuming https://github.com/tc39/proposal-string-matchall/pull/33 will be merged.

Modify $262.uncallableAndIsHTMLDDA() to an optional $262.IsHTMLDDA (whose use must be guarded by a feature of the same name), and narrowly/correctly prescribe its requirements consistent with `document.all`'s behavior in HTML. 

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`.

A recent commit introduced a document that enumerated acceptable values
for the test "features" metadata tag. However, this list was incomplete,
and maintaining it placed extra burden on the project owners.

Restructure the document into a machine-readable format. Add entries for
all previously-omitted values. Add in-line documentation with
recommendations for maintenance of the file. Extend the project's
linting tool to validate tests according to the document's contents.