Add missing import of 'subunit' in test-requirements.txt
Add missing import of 'subunit' in test-requirements.txt

 tools/colorized.py import subunit
 however test-requirements.txt doesn't note this.

 Closes-Bug: #1224155

Change-Id: I4e1a8ee449c4fcf733116809de41e86d10f17e85

vedujoshi/os_tempest,varunarya10/tempest,hpcloud-mon/tempest,openstack/tempest,tudorvio/tempest,danielmellado/tempest,Lilywei123/tempest,dkalashnik/tempest,ntymtsiv/tempest,armando-migliaccio/tempest,masayukig/tempest,afaheem88/tempest_neutron,Juniper/tempest,tonyli71/tempest,LIS/lis-tempest,varunarya10/tempest,dkalashnik/tempest,queria/my-tempest,alinbalutoiu/tempest,queria/my-tempest,Juraci/tempest,redhat-cip/tempest,yamt/tempest,cloudbase/lis-tempest,rakeshmi/tempest,BeenzSyed/tempest,danielmellado/tempest,hayderimran7/tempest,manasi24/tempest,xbezdick/tempest,izadorozhna/tempest,armando-migliaccio/tempest,cloudbase/lis-tempest,alinbalutoiu/tempest,izadorozhna/tempest,roopali8/tempest,rzarzynski/tempest,eggmaster/tempest,bigswitch/tempest,NexusIS/tempest,cisco-openstack/tempest,citrix-openstack-build/tempest,vedujoshi/tempest,akash1808/tempest,rakeshmi/tempest,FujitsuEnablingSoftwareTechnologyGmbH/tempest,sebrandon1/tempest,flyingfish007/tempest,adkerr/tempest,yamt/tempest,CiscoSystems/tempest,pczerkas/tempest,pczerkas/tempest,vmahuli/tempest,LIS/lis-tempest,FujitsuEnablingSoftwareTechnologyGmbH/tempest,Vaidyanath/tempest,itskewpie/tempest,hpcloud-mon/tempest,Mirantis/tempest,ntymtsiv/tempest,jaspreetw/tempest,akash1808/tempest,Mirantis/tempest,ebagdasa/tempest,JioCloud/tempest,tudorvio/tempest,manasi24/tempest,eggmaster/tempest,Juraci/tempest,Juniper/tempest,cisco-openstack/tempest,neerja28/Tempest,flyingfish007/tempest,itskewpie/tempest,masayukig/tempest,adkerr/tempest,JioCloud/tempest,vmahuli/tempest,BeenzSyed/tempest,citrix-openstack/build-tempest,neerja28/Tempest,ebagdasa/tempest,pandeyop/tempest,bigswitch/tempest,pandeyop/tempest,Vaidyanath/tempest,Tesora/tesora-tempest,tonyli71/tempest,jaspreetw/tempest,xbezdick/tempest,nunogt/tempest,nunogt/tempest,manasi24/jiocloud-tempest-qatempest,vedujoshi/tempest,vedujoshi/os_tempest,jamielennox/tempest,manasi24/jiocloud-tempest-qatempest,sebrandon1/tempest,zsoltdudas/lis-tempest,zsoltdudas/lis-tempest,Tesora/tesora-tempest,hayderimran7/tempest,afaheem88/tempest,citrix-openstack-build/tempest,openstack/tempest,NexusIS/tempest,CiscoSystems/tempest,Lilywei123/tempest,afaheem88/tempest,afaheem88/tempest_neutron,rzarzynski/tempest,roopali8/tempest,redhat-cip/tempest,citrix-openstack/build-tempest,jamielennox/tempest