[taskcluster] Actually use the new image in tests

This is a follow-up to web-platform-tests#23190. I forgot that tools/ci/tc/tasks/test.yml
existed, so we really only updated the image of the decision task.

Drive-by:
* Update README to document this.
* Remove python3-pip from `install` as it's now included in the image.

WASM JS API: Test new Web IDL class string behavior

Follows whatwg/webidl#357.

Move hit-test idlharness to webxr/hit-test (web-platform-tests#23204)

Fix and export svg/text/textpath-local-url-reference.html to WPT

Since the <script> tag appeared after defining the base URL of the
document, the script wasn't fetched and run. This likely worked out
anyway because of having Ahem available through other means.
Fix this and export the test to WPT while at it.

Change-Id: I9dff35db0bcfa6ccaa697d791583e897e47b768f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2162888
Reviewed-by: Stephen Chenney <[email protected]>
Commit-Queue: Fredrik Söderquist <[email protected]>
Cr-Commit-Position: refs/heads/master@{#761952}

Add a test checking that deviceId is empty before getUserMedia is cal…

…led (web-platform-tests#23202)

* Add a test checking that deviceId is empty before getUserMedia is called

* Add test for number of exposed devices before capture

Update interfaces/cssom.idl (web-platform-tests#23197)

Source: https://github.com/tidoust/reffy-reports/blob/c0d151a/ed/idl/cssom.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/678477177

Re-land: "Do main-thread layer update even if hidden_by_backface_visi…

…bility""

This reverts commit 2b5c4137bacc50e5f3a925081b648b6aea43e9c7.

The fix is to wait one more frame. This makes the test pass reliably
on debug builds.

Bug: 977658

Change-Id: Ie8a70e6043e92267f99022cdc78b7ca687370cec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2161454
Reviewed-by: Robert Flack <[email protected]>
Commit-Queue: Chris Harrelson <[email protected]>
Cr-Commit-Position: refs/heads/master@{#761978}

Add a test for declarative Shadow DOM after attachShadow()

This is a new test to verify that declarative shadow DOM (properly)
fails if attachShadow() has already been called on the host node.

Bug: 1042130
Change-Id: Ibca94f9be5f6a1eb3c3bdd491f84e782e8cc6d9f

Add python3 and python3-pip to the docker image (web-platform-tests#2…

…3190)

See
web-platform-tests#23182 (comment)

Update README too

Add assert_precondition to the lint (web-platform-tests#23189)

This is deprecated in favour of assert_implements and
assert_implements_optional. There are a few files left using
assert_precondition, added to the lint ignorelist.