Update web-platform-tests docs

docs/testing/writing_layout_tests.md

 # Writing Layout Tests
 
 _Layout tests_ is a bit of a misnomer. This term is
 [a part of our WebKit heritage](https://webkit.org/blog/1452/layout-tests-theory/),
 and we use it to refer to every test that is written as a Web page (HTML, SVG,
 or XHTML) and lives in
 [third_party/WebKit/LayoutTests/](../../third_party/WebKit/LayoutTests).
 
 [TOC]
 
@@ skipping 194 matching lines @@
 cost and benefits of having the tests
 [gracefully degrade to manual tests](./layout_tests_with_manual_fallback.md) in
 the absence of the testing APIs.
 
 *** promo
 In many cases, the user gesture is not actually necessary. For example, many
 event handling tests can use
 [synthetic events](https://developer.mozilla.org/docs/Web/Guide/Events/Creating_and_triggering_events).
 ***
 
-*** note
-TODO: document wpt_automation. Manual tests might end up moving here.
-***
-
 ### Relying on Blink-Specific Testing APIs
 
 Tests that cannot be expressed using the Web Platform APIs or WPT's testing APIs
 use Blink-specific testing APIs. These APIs are only available in
 [content_shell](./layout_tests_in_content_shell.md), and should only be used as
 a last resort.
 
 A downside of Blink-specific APIs is that they are not as well documented as the
 Web Platform features. Learning to use a Blink-specific feature requires finding
 other tests that use it, or reading its source code.
@@ skipping 350 matching lines @@
 * The `http/` directory hosts tests that require an HTTP server (see above).
 * The `resources/` subdirectory in every directory contains binary files, such
   as media files, and code that is shared by multiple test files.
 
 *** note
 Some layout tests consist of a minimal HTML page that references a JavaScript
 file in `resources/`. Please do not use this pattern for new tests, as it goes
 against the minimality principle. JavaScript and CSS files should only live in
 `resources/` if they are shared by at least two test files.
 ***