integration/firestore/gulpfile.js: add testing_hooks_util.ts to the list of copied files

This may fix the following error:

```
[tsl] ERROR in /home/runner/work/firebase-js-sdk/firebase-js-sdk/integration/firestore/temp/test/integration/api/query.test.ts(76,50)
TS2307: Cannot find module '../util/testing_hooks_util' or its corresponding type declarations.
```

https://github.com/firebase/firebase-js-sdk/actions/runs/4351236611/jobs/7602696515

Author: dconeybe

integration/firestore/gulpfile.js

@@ -45,6 +45,7 @@ function copyTests() {
         testBase + '/integration/util/events_accumulator.ts',
         testBase + '/integration/util/helpers.ts',
         testBase + '/integration/util/settings.ts',
+        testBase + '/integration/util/testing_hooks_util.ts',
         testBase + '/util/equality_matcher.ts',
         testBase + '/util/promise.ts'
       ],