Add iterpath and iterdir to CSS test runner.
Add iterpath and iterdir to CSS test runner.

This allows running a subset of the CSS tests again, by specifying
a directory or single test.

Fixes #18931.

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