This pref is required by the common animation interpolation test function used
in many of the CSS test suites but currently we only enable it on Nightly. As a
result, each time a new spec adds animation tests, we get failures on our beta
simulations. Rather than turn this on for each directory it is used in each
time a new failure occurs, this patch makes us set it globally.

This patch also removes a number of instances where we set this pref locally.
It only removes the instances that were specifically added due to the
introduction of the common CSS interpolation test function. The existing
instances remain untouched since they serve as documentation of other places
where we are specifically testing that feature.

Differential Revision: https://phabricator.services.mozilla.com/D47034

--HG--
extra : moz-landing-system : lando