* Refuse to serialize some combinations of values that the shorthand
can not represent: 'grid-template-areas: (not none)' combined with
'grid-template-rows: subgrid' or 'grid-template-columns: subgrid'.
(The former used to cause an assertion failure.)
* Remove an extraneous trailing space that occured when a <track-list>
was last. (ie. followed by an omitted <line-names>)
* Add tests for the result of this serialization.
