testutils: allow concurrent serve with httputils
testutils: allow concurrent serve with httputils

Use ServeMux instead of Server which allows concurrent listeners registered for
the same path.

intfrr/rkt,s-urbaniak/rkt,Wilybald/rkt,lee212/rkt,yifan-gu/rocket,shobhit85/rkt,Shashwatsh/rkt,yifan-gu/rocket,Wilybald/rkt,sgotti/rkt,jonboulle/rkt,smarvin/rkt,markllama/rkt,kbrwn/rkt,yifan-gu/rkt,sgotti/rkt,yifan-gu/rkt,mjg59/rkt,kbrwn/rkt,euank/rkt,squeed/rkt,alexhersh/rkt,rkt/rkt,sgotti/rkt,Shashwatsh/rkt,jjlakis/rkt,liubq919/rkt,mike1729/rkt,coreos/rkt,liugenping/rkt,luebken/rkt,nhlfr/rkt,joshix/rkt,Wilybald/rkt,lucasallan/rkt,ybubnov/rkt,mjg59/rkt,joshix/rkt,il9ue/rkt,dgonyeo/rkt,jakon89/rkt,s-urbaniak/rkt,kinvolk/rkt,dalanlan/rkt,jzelinskie/rkt,luebken/rkt,shobhit85/rkt,joshix/rkt,mjg59/rkt,tmrts/rkt,jzelinskie/rkt,iankronquist/rkt,intelsdi-x/rkt,markllama/rkt,monstermunchkin/rkt,jasonchaffee/rkt,matthaias/rkt,jakon89/rkt,cab105/rkt,joshix/rkt,il9ue/rkt,luebken/rkt,krnowak/rkt,marineam/rkt,endocode/rkt,alban/rkt,dalanlan/rkt,MattDees/rkt,rkt/rkt,intelsdi-x/rkt,tmrts/rkt,chrisseto/rkt,kinvolk/rkt,rkt/rkt,smarvin/rkt,liugenping/rkt,yifan-gu/rkt,kinvolk/rkt,lee212/rkt,kbrwn/rkt,jasonchaffee/rkt,blixtra/rkt,liubq919/rkt,monstermunchkin/rkt,alban/rkt,adfernandes/rkt,lucasallan/rkt,iaguis/rkt,alexhersh/rkt,markllama/rkt,mike1729/rkt,tv42/rkt,tv42/rkt,tv42/rkt,matthaias/rkt,sstabellini/rkt,marineam/rkt,jjlakis/rkt,iankronquist/rkt,cab105/rkt,intelsdi-x/rkt,MattDees/rkt,euank/rkt,linki/rkt,krnowak/rkt,endocode/rkt,linki/rkt,Shashwatsh/rkt,squeed/rkt,blixtra/rkt,nhlfr/rkt,monstermunchkin/rkt,il9ue/rkt,marineam/rkt,jonboulle/rkt,liugenping/rkt,Yhgenomics/rkt,adfernandes/rkt,linki/rkt,akshaykarle/rkt,tmrts/rkt,smarvin/rkt,achanda/rkt,blixtra/rkt,cab105/rkt,alexhersh/rkt,dgonyeo/rkt,shobhit85/rkt,ybubnov/rkt,jjlakis/rkt,jonboulle/rkt,Yhgenomics/rkt,chrisseto/rkt,sstabellini/rkt,jasonchaffee/rkt,lee212/rkt,lucasallan/rkt,jzelinskie/rkt,rkt/rkt,intfrr/rkt,Yhgenomics/rkt,sstabellini/rkt,akshaykarle/rkt,jakon89/rkt,iaguis/rkt,alban/rkt,sinfomicien/rkt,yifan-gu/rocket,nhlfr/rkt,iankronquist/rkt,lucasallan/rkt,achanda/rkt,endocode/rkt,sinfomicien/rkt,coreos/rkt,liubq919/rkt,coreos/rkt,adfernandes/rkt,Wilybald/rkt,akshaykarle/rkt,euank/rkt,dalanlan/rkt,krnowak/rkt,squeed/rkt,MattDees/rkt,sinfomicien/rkt,s-urbaniak/rkt,mike1729/rkt,matthaias/rkt,intfrr/rkt,iaguis/rkt,chrisseto/rkt,dgonyeo/rkt,kinvolk/rkt,achanda/rkt,ybubnov/rkt