Add `shouldIgnoreViolation` optional prop to Scenario for a11y tests

[acelaya]

Context

We have recently found an accessibility test that was failing dues to axe-core not allowing a very specific combination of elements, specifically a button with combobox role linked with a label using its for attribute.

While the specification says this is a valid combination, there's a specific screen reader which does not play well with it, and that's why axe-core reports a violation.

We have tested the combination with other screen readers (VoiceOver and orca), and the elements get announced as expected, so we want to be able to ignore this violation.

Changes

This PR introduces a new shouldIgnoreViolation callback that can be provided as part of every scenario passed to checkAccessibility.

This callback is invoked with every violation reported by axe-core, allowing downstream code to determine which violations are relevant and which are not.

I decided to use this approach instead of just providing the ids of violations to ignore, because then we could end up ignoring actual violations by mistake.

With this, downstream code has all the information about the violation in order to decide whether it has to be ignored or not.

How to use

As a POC, I have used this PR in client in the test where this was first spotted, so it will look more or less like this:

-  it(
-    'should pass a11y checks',
-    checkAccessibility({
-      content: () => createComponent(),
-    }),
-  );
+  it('should pass a11y checks', async () => {
+    const wrapper = createComponent();
+    const select = await waitForElement(wrapper, SelectNext);
+
+    await checkAccessibility({
+      content: () => wrapper,
+      shouldIgnoreViolation: ({ id, nodes }) => {
+        if (id !== 'button-name') {
+          return false;
+        }
+
+        // axe-core can report a violation on any button[role="combobox"] linked
+        // to a label if it does not have aria-label or aria-labelledby, because
+        // the Dragon NaturallySpeaking screen reader does not play well with
+        // that combination.
+        // Since its use is marginal, we want to ignore the "button-name"
+        // violation if reported on SelectNext.
+        // See https://github.com/dequelabs/axe-core/issues/4235 for context
+
+        const targets = nodes.flatMap(node => node.target);
+        return targets.includes(`#${select.prop('buttonId')}`);
+      },
+    })();
+  });