Log remaining govuk-<SOMETHING>() calls when test fails

The test detecting if any `govuk-<SOMETHING>()` calls make it
to the CSS output was only telling you a function made it through
 (likely through a typo). That's very little information to go by
for finding why that function made it through.

This new test checks the length of the regex matches, allowing
Jest to output the list of matches if the test fails:

```
    Expected length: 0
    Received length: 1
    Received array:  [["govuk-color(\"black\")"]]
```

Author: romaricpascal

packages/govuk-frontend/src/govuk/all.unit.test.mjs

@@ -94,9 +94,12 @@ describe('GOV.UK Frontend', () => {
   it('does not contain any unexpected govuk- function calls', async () => {
     const sass = '@import "index"'
 
-    await expect(compileSassString(sass)).resolves.toMatchObject({
-      css: expect.not.stringMatching(/_?govuk-[\w-]+\(.*?\)/g)
-    })
+    const { css } = await compileSassString(sass)
+    const matches = css.matchAll(/_?govuk-[\w-]+\(.*?\)/g)
+
+    // `matchAll` does not return an actual `Array` so we need
+    // a little conversion before we can check its length
+    expect(Array.from(matches)).toHaveLength(0)
   })
 
   describe('Sass documentation', () => {