test: prevent invalid writeheader code 0 during tests

What does this MR do?

If the pages handler inside TestDomainResolverError\timeout finishes before the subtests then w.WriteHeader(test.status) is called with test.status being 0.

That throws an invalid writeheader error.

TODO

• I added the Changelog trailer (e.g. Changelog: added) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines