Add two font shorthand serialization tests
Add two font shorthand serialization tests

Adds tests:

- font_should_serialize_to_empty_if_there_are_nondefault_subproperties

Tests that the font subproperty serializes to an empty string when
certain non-default subproperties are defined.

- font_should_serialize_all_available_properties

Verifies that all properties that the font shorthand can display
will be serialized. This test was previously commented out and has
been simplified.

SimonSapin/servo,mattnenterprise/servo,notriddle/servo,paulrouget/servo,thiagopnts/servo,peterjoel/servo,dsandeephegde/servo,notriddle/servo,KiChjang/servo,DominoTree/servo,pyfisch/servo,avadacatavra/servo,fiji-flo/servo,mattnenterprise/servo,saneyuki/servo,emilio/servo,SimonSapin/servo,szeged/servo,eddyb/servo,rnestler/servo,DominoTree/servo,dsandeephegde/servo,KiChjang/servo,canaltinova/servo,eddyb/servo,dsandeephegde/servo,avadacatavra/servo,larsbergstrom/servo,eddyb/servo,rnestler/servo,splav/servo,fiji-flo/servo,saneyuki/servo,anthgur/servo,splav/servo,avadacatavra/servo,larsbergstrom/servo,pyfisch/servo,jimberlage/servo,splav/servo,dati91/servo,ConnorGBrewster/servo,mattnenterprise/servo,ConnorGBrewster/servo,dsandeephegde/servo,sadmansk/servo,saneyuki/servo,KiChjang/servo,nnethercote/servo,eddyb/servo,fiji-flo/servo,dati91/servo,thiagopnts/servo,pyfisch/servo,DominoTree/servo,fiji-flo/servo,pyfisch/servo,szeged/servo,anthgur/servo,DominoTree/servo,emilio/servo,sadmansk/servo,SimonSapin/servo,cbrewster/servo,canaltinova/servo,pyfisch/servo,rnestler/servo,eddyb/servo,paulrouget/servo,szeged/servo,jimberlage/servo,peterjoel/servo,cbrewster/servo,splav/servo,nnethercote/servo,thiagopnts/servo,saneyuki/servo,avadacatavra/servo,notriddle/servo,jimberlage/servo,KiChjang/servo,splav/servo,paulrouget/servo,notriddle/servo,KiChjang/servo,nnethercote/servo,emilio/servo,nnethercote/servo,anthgur/servo,dati91/servo,ConnorGBrewster/servo,peterjoel/servo,larsbergstrom/servo,avadacatavra/servo,upsuper/servo,notriddle/servo,sadmansk/servo,DominoTree/servo,mattnenterprise/servo,thiagopnts/servo,emilio/servo,anthgur/servo,rnestler/servo,saneyuki/servo,dati91/servo,larsbergstrom/servo,canaltinova/servo,paulrouget/servo,cbrewster/servo,rnestler/servo,larsbergstrom/servo,CJ8664/servo,mbrubeck/servo,KiChjang/servo,paulrouget/servo,KiChjang/servo,paulrouget/servo,sadmansk/servo,CJ8664/servo,paulrouget/servo,szeged/servo,mbrubeck/servo,CJ8664/servo,emilio/servo,thiagopnts/servo,mbrubeck/servo,upsuper/servo,dati91/servo,mbrubeck/servo,fiji-flo/servo,paulrouget/servo,jimberlage/servo,CJ8664/servo,pyfisch/servo,anthgur/servo,emilio/servo,upsuper/servo,upsuper/servo,eddyb/servo,peterjoel/servo,upsuper/servo,emilio/servo,mattnenterprise/servo,eddyb/servo,notriddle/servo,jimberlage/servo,larsbergstrom/servo,paulrouget/servo,ConnorGBrewster/servo,fiji-flo/servo,saneyuki/servo,sadmansk/servo,mbrubeck/servo,thiagopnts/servo,canaltinova/servo,canaltinova/servo,cbrewster/servo,rnestler/servo,mattnenterprise/servo,splav/servo,avadacatavra/servo,canaltinova/servo,peterjoel/servo,dsandeephegde/servo,SimonSapin/servo,DominoTree/servo,larsbergstrom/servo,splav/servo,pyfisch/servo,emilio/servo,szeged/servo,nnethercote/servo,szeged/servo,notriddle/servo,DominoTree/servo,jimberlage/servo,dsandeephegde/servo,dati91/servo,peterjoel/servo,splav/servo,jimberlage/servo,nnethercote/servo,notriddle/servo,KiChjang/servo,szeged/servo,dsandeephegde/servo,mattnenterprise/servo,nnethercote/servo,CJ8664/servo,nnethercote/servo,saneyuki/servo,nnethercote/servo,cbrewster/servo,mbrubeck/servo,avadacatavra/servo,splav/servo,SimonSapin/servo,CJ8664/servo,CJ8664/servo,peterjoel/servo,upsuper/servo,cbrewster/servo,sadmansk/servo,cbrewster/servo,KiChjang/servo,saneyuki/servo,cbrewster/servo,nnethercote/servo,rnestler/servo,splav/servo,dsandeephegde/servo,dati91/servo,jimberlage/servo,ConnorGBrewster/servo,rnestler/servo,emilio/servo,larsbergstrom/servo,mbrubeck/servo,thiagopnts/servo,ConnorGBrewster/servo,DominoTree/servo,thiagopnts/servo,anthgur/servo,emilio/servo,szeged/servo,canaltinova/servo,mattnenterprise/servo,peterjoel/servo,ConnorGBrewster/servo,szeged/servo,peterjoel/servo,SimonSapin/servo,pyfisch/servo,mbrubeck/servo,sadmansk/servo,jimberlage/servo,larsbergstrom/servo,peterjoel/servo,DominoTree/servo,KiChjang/servo,SimonSapin/servo,saneyuki/servo,paulrouget/servo,fiji-flo/servo,pyfisch/servo,SimonSapin/servo,szeged/servo,notriddle/servo,notriddle/servo,pyfisch/servo,DominoTree/servo,upsuper/servo,fiji-flo/servo,CJ8664/servo,ConnorGBrewster/servo,upsuper/servo,anthgur/servo,sadmansk/servo,larsbergstrom/servo,dati91/servo,eddyb/servo,avadacatavra/servo,anthgur/servo,canaltinova/servo,saneyuki/servo