Use document.getElementById to check target exists

Because the string passed to `querySelector` is evaluated as a CSS selector, this can fail if the ID contains characters that have a special meaning in CSS, such as `.` or `[]` – the ID would need to be escaped for it to be evaluated correctly.

Avoid this by using `document.getElementById` instead, so the ID is no longer evaluated as a selector.

The downside is that we can't constrain the scope to the $module. However, we don't think this should cause any issues as the ID _should_ be unique within the document.

This fixes the failing tests introduced in the previous commit.

src/govuk/components/checkboxes/checkboxes.js

@@ -32,7 +32,7 @@ Checkboxes.prototype.init = function () {
 
     // Skip checkboxes without data-aria-controls attributes, or where the
     // target element does not exist.
-    if (!target || !$module.querySelector('#' + target)) {
+    if (!target || !document.getElementById(target)) {
       return
     }
 

src/govuk/components/radios/radios.js

@@ -32,7 +32,7 @@ Radios.prototype.init = function () {
 
     // Skip radios without data-aria-controls attributes, or where the
     // target element does not exist.
-    if (!target || !$module.querySelector('#' + target)) {
+    if (!target || !document.getElementById(target)) {
       return
     }