Add failing test

This test fails with:

```
Character count › when JavaScript is available › when counting characters › when the ID starts with a number › still works correctly

    DOMException: Failed to execute 'querySelector' on 'Element': '[id=1_more-detail-info]' is not a valid selector.
```

which matches the console error reported in the issue [1]

[1]: #2058

src/govuk/components/character-count/character-count.test.js

@@ -141,6 +141,16 @@ describe('Character count', () => {
           expect(ariaHidden).toBeFalsy()
         })
       })
+
+      describe('when the ID starts with a number', () => {
+        it('still works correctly', async () => {
+          await goToExample('with-id-starting-with-number')
+
+          const message = await page.$eval('.govuk-character-count__message', el => el.innerHTML.trim())
+
+          expect(message).toEqual('You have 10 characters remaining')
+        })
+      })
     })
 
     describe('when counting words', () => {

src/govuk/components/character-count/character-count.yaml

@@ -223,3 +223,11 @@ examples:
       classes: app-character-count--custom-modifier
       errorMessage:
         text: Error message
+  - name: with id starting with number
+    hidden: true
+    data:
+      name: more-detail
+      id: 1_more-detail
+      maxlength: 10
+      label:
+        text: Can you provide more detail?