as documented in
https://web-platform-tests.org/writing-tests/testharness-api.html#setup

On errors without setup(), the harness would assume a single page test and
report as FAIL in an imaginary subtest name based on document.title.
Wrapping in setup() leads to failures reporting as a toplevel ERROR, which
perhaps better reports the nature of the failure.

Differential Revision: https://phabricator.services.mozilla.com/D51052

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1592503
gecko-commit: 02d15f78c542fb761dbf46f645c2fd50e47869db
gecko-integration-branch: autoland
gecko-reviewers: padenot