When parsing values in `process_a_keyframes_argument` we don't expand
properties using `StyleComputer::for_each_property_expanding_shorthands`
unlike most other places - this means that if we parse a `border` we end
up with the `border`'s sub-properties (`border-width`, `border-style`,
`border-color`) still in their unexpanded forms (`CSSKeywordValue`,
`LengthStyleValue`, `StyleValueList`, etc) rather than
`ShorthandStyleValue`s which causes a crash when serializing the
`border` value in `KeyframeEffect::get_keyframes`.
The proper fix here is to parse `border`'s sub-properties directly to
`ShorthandStyleValue`s instead of relying on
`StyleComputer::for_each_property_expanding_shorthand` to do this
conversion for us but this may be a while off.
This commit also imports the previously crashing tests.
