Bug 1509418 - Enable the feature in Nightly. r=heycam,boris

There are a bunch of missing tests, and there are some tests that don't
match the current spec text, that I need to write or fix before I enable the
feature everywhere.

But there are fairly complex invalidation tests, that we pass flawlessly :)

Differential Revision: https://phabricator.services.mozilla.com/D71424

modules/libpref/init/StaticPrefList.yaml

@@ -6050,7 +6050,7 @@
 # Is support for :is() and :where() enabled?
 - name: layout.css.is-where-selectors.enabled
   type: RelaxedAtomicBool
-  value: false
+  value: @IS_NIGHTLY_BUILD@
   mirror: always
   rust: true
 

testing/web-platform/meta/css/selectors/__dir__.ini

@@ -1 +1 @@
-prefs: [layout.css.focus-visible.enabled:true]
+prefs: [layout.css.focus-visible.enabled:true,layout.css.is-where-selectors.enabled:true]

testing/web-platform/meta/css/selectors/invalidation/is.html.ini

@@ -1,16 +0,0 @@
-[is.html]
-  [Invalidate :is() for compound selector arguments.]
-    expected: FAIL
-
-  [Test specificity of :is().]
-    expected: FAIL
-
-  [Invalidate :is() for complex selector arguments.]
-    expected: FAIL
-
-  [Invalidate :is() for simple selector arguments.]
-    expected: FAIL
-
-  [Invalidate nested :is().]
-    expected: FAIL
-

testing/web-platform/meta/css/selectors/invalidation/where.html.ini

@@ -1,16 +0,0 @@
-[where.html]
-  [Invalidate :where() for complex selector arguments.]
-    expected: FAIL
-
-  [Preconditions.]
-    expected: FAIL
-
-  [Invalidate :where() for simple selector arguments.]
-    expected: FAIL
-
-  [Invalidate :where() for compound selector arguments.]
-    expected: FAIL
-
-  [Invalidate nested :where().]
-    expected: FAIL
-