MDL-82373 behat: Disable axe target-size rule

Disable the target-size rule.
This rule was added in Axe 4.7, but changed in 4.8 to allow for spacing
around the element. See https://github.com/dequelabs/axe-core/pull/4117
for more information on this change.

Moodle 4.2 has an earlier version of Axe which does not have this rule.
Moodle 4.4 has a later, unaffected, version of Axe.

The simplest solution here is to simply disable the buggy rule.

lib/tests/behat/behat_accessibility.php

@@ -218,6 +218,14 @@ EOF;
                 'type' > 'tag',
                 'values' => array_merge($standardtags, $extratags),
             ],
+            'rules' => [
+                // Disable the target-size rule.
+                // This rule was added in Axe 4.7, but changed in 4.8 to allow for spacing around the element.
+                // See https://github.com/dequelabs/axe-core/pull/4117 for more information on this change.
+                'target-size' => [
+                    'enabled' => false,
+                ],
+            ]
         ]);
     }
 }