fix(autocomplete-valid): treat values "xon" and "xoff" as non-WCAG-violations (#4878)

Similar to #3188. Resolves a false positive report from an auditor
customer.

Closes: #4877

Author: dbjorge

lib/checks/forms/autocomplete-valid.json

@@ -17,7 +17,9 @@
       "disabled",
       "enabled",
       "undefined",
-      "null"
+      "null",
+      "xoff",
+      "xon"
     ],
     "ignoredValues": ["text", "pronouns", "gender", "message", "content"]
   }

test/integration/rules/autocomplete-valid/autocomplete-valid.html

@@ -164,12 +164,14 @@
 <input autocomplete="enabled" id="pass88" />
 <input autocomplete="undefined" id="pass89" />
 <input autocomplete="null" id="pass90" />
+<input autocomplete="xon" id="pass91" />
+<input autocomplete="xoff" id="pass92" />
 
 <!-- Webauthn tokens -->
-<input autocomplete="email webauthn" id="pass91" />
-<input autocomplete="section-foo email webauthn" id="pass92" />
-<input autocomplete="section-foo work email webauthn" id="pass93" />
-<input autocomplete="section-foo billing work email webauthn" id="pass94" />
+<input autocomplete="email webauthn" id="pass93" />
+<input autocomplete="section-foo email webauthn" id="pass94" />
+<input autocomplete="section-foo work email webauthn" id="pass95" />
+<input autocomplete="section-foo billing work email webauthn" id="pass96" />
 
 <!-- Incomplete, ignored values -->
 <input autocomplete="text" id="incomplete1" />

test/integration/rules/autocomplete-valid/autocomplete-valid.json

@@ -103,6 +103,8 @@
     ["#pass91"],
     ["#pass92"],
     ["#pass93"],
-    ["#pass94"]
+    ["#pass94"],
+    ["#pass95"],
+    ["#pass96"]
   ]
 }