Tests: Import all WPT css/css-nesting tests

A few are skipped for now:
- A few ref tests fail
- Crash tests are not supported by our runner and time out
- top-level-is-scope.html crashes and needs further investigation

Tests/LibWeb/Text/input/wpt-import/common/gc.js

@@ -0,0 +1,52 @@
+/**
+ * Does a best-effort attempt at invoking garbage collection. Attempts to use
+ * the standardized `TestUtils.gc()` function, but falls back to other
+ * environment-specific nonstandard functions, with a final result of just
+ * creating a lot of garbage (in which case you will get a console warning).
+ *
+ * This should generally only be used to attempt to trigger bugs and crashes
+ * inside tests, i.e. cases where if garbage collection happened, then this
+ * should not trigger some misbehavior. You cannot rely on garbage collection
+ * successfully trigger, or that any particular unreachable object will be
+ * collected.
+ *
+ * @returns {Promise<undefined>} A promise you should await to ensure garbage
+ * collection has had a chance to complete.
+ */
+self.garbageCollect = async () => {
+  // https://testutils.spec.whatwg.org/#the-testutils-namespace
+  if (self.TestUtils?.gc) {
+    return TestUtils.gc();
+  }
+
+  // Use --expose_gc for V8 (and Node.js)
+  // to pass this flag at chrome launch use: --js-flags="--expose-gc"
+  // Exposed in SpiderMonkey shell as well
+  if (self.gc) {
+    return self.gc();
+  }
+
+  // Present in some WebKit development environments
+  if (self.GCController) {
+    return GCController.collect();
+  }
+
+  console.warn(
+    'Tests are running without the ability to do manual garbage collection. ' +
+    'They will still work, but coverage will be suboptimal.');
+
+  for (var i = 0; i < 1000; i++) {
+    gcRec(10);
+  }
+
+  function gcRec(n) {
+    if (n < 1) {
+      return {};
+    }
+
+    let temp = { i: "ab" + i + i / 100000 };
+    temp += "foo";
+
+    gcRec(n - 1);
+  }
+};