Tests fail without internet access

[mithro]

Was trying to work on test infrastructure while on a plane and found the following problems;

• We depend on webfonts, which only work when online
• We depend on a WebM video hosted http://video.webmfiles.org/big-buck-bunny_trailer.webm

Both seem reasonable (we probably don't want a giant webm file in our repository) so I'm not sure what the best solution it.

[steveblock]

I'm removing the dependence on the external video in https://github.com/web-animations/web-animations-js/pull/316