Add tests for active descendant a11y rule

test/validator/samples/a11y-aria-activedescendant/input.svelte

@@ -0,0 +1,17 @@
+<!-- VALID -->
+<input />
+<input tabindex="0" />
+<input aria-activedescendant="some-id" />
+<input aria-activedescendant="some-id" tabindex={0} />
+<input aria-activedescendant="some-id" tabindex={1} />
+<input aria-activedescendant="some-id" tabindex="0" />
+<input aria-activedescendant="some-id" tabindex={-1} />
+<input aria-activedescendant="some-id" tabindex="-1" />
+
+<div />
+<div aria-activedescendant="some-id" role="tablist" tabindex={-1} />
+<div aria-activedescendant="some-id" role="tablist" tabindex="-1" />
+
+<!-- INVALID -->
+<div aria-activedescendant="some-id" />
+

test/validator/samples/a11y-aria-activedescendant/warnings.json

@@ -0,0 +1,17 @@
+[
+  {
+    "code": "a11y-aria-activedescendant-has-tabindex",
+    "end": {
+      "character": 568,
+      "column": 36,
+      "line": 16
+    },
+    "message": "A11y: Elements with attribute aria-activedescendant should have tabindex value",
+    "pos": 537,
+    "start": {
+      "character": 537,
+      "column": 5,
+      "line": 16
+    }
+  }
+]