In aa48a2c I added a timeout for paint
worklet threads. However, the test was broken. The blocking sleep
function that was implemented as part of that commit was guarded behind
the `dom.worklet.blockingsleep.enabled` pref, and while I ran the
wpt-tests with that pref enabled, the test runner for sure did not.
I tried running the test _without_ the pref enabled, and the tests still
pass. This is because even the reference in that reftest is that of
a broken image background, and *both* the paintworklet thread timing out
and `sleep()` not being in scope would render the same thing: a broken
image, which compares equal to the reference.

This patch makes sure that now the pref is enabled for wpt worklet
tests, and that such we can distinguish an actual timeout (test pass)
from an unexpected situation (when we should fail the test).