Automatic update from web-platform-tests
[FileAPI] Opt-in to single-page test feature (#19925)

testharness.js was recently extended with an API to explicitly opt-in to
the "single page test" feature [1]. As per WPT RFC 28 [2], tests which
do not use this API and which do not declare any subtests will soon be
reported as a harness error.

Update the tests which previously opted in implicitly to use the new
API.

[1] https://github.com/web-platform-tests/wpt/pull/19449

[2] https://github.com/web-platform-tests/rfcs/blob/master/rfcs/single_test.md

wpt-commits: 89ebe927c237ed05c732490ca7d82b7fc41f5908
wpt-pr: 19925