test_fork_error: Fix incorrect test mock
test_fork_error: Fix incorrect test mock

The previous os.fork mock returned a *function* that returned an OSError
object.  Oops.

Change-Id: I6e5dff930cf5614f7061efdfd8429963703cc214

dims/neutron,dhanunjaya/neutron,jerryz1982/neutron,takeshineshiro/neutron,noironetworks/neutron,klmitch/neutron,igor-toga/local-snat,sasukeh/neutron,jumpojoy/neutron,mmnelemane/neutron,noironetworks/neutron,openstack/neutron,glove747/liberty-neutron,asgard-lab/neutron,neoareslinux/neutron,silenci/neutron,huntxu/neutron,watonyweng/neutron,openstack/neutron,yanheven/neutron,JioCloud/neutron,mahak/neutron,glove747/liberty-neutron,suneeth51/neutron,dhanunjaya/neutron,javaos74/neutron,wolverineav/neutron,watonyweng/neutron,jacknjzhou/neutron,bigswitch/neutron,suneeth51/neutron,shahbazn/neutron,JioCloud/neutron,javaos74/neutron,bgxavier/neutron,sebrandon1/neutron,takeshineshiro/neutron,chitr/neutron,jerryz1982/neutron,jumpojoy/neutron,adelina-t/neutron,paninetworks/neutron,mmnelemane/neutron,barnsnake351/neutron,openstack/neutron,adelina-t/neutron,vivekanand1101/neutron,neoareslinux/neutron,vivekanand1101/neutron,cisco-openstack/neutron,apporc/neutron,barnsnake351/neutron,swdream/neutron,JianyuWang/neutron,swdream/neutron,mahak/neutron,bgxavier/neutron,eayunstack/neutron,silenci/neutron,asgard-lab/neutron,klmitch/neutron,eayunstack/neutron,JianyuWang/neutron,cisco-openstack/neutron,sasukeh/neutron,shahbazn/neutron,jacknjzhou/neutron,NeCTAR-RC/neutron,igor-toga/local-snat,SamYaple/neutron,eonpatapon/neutron,mandeepdhami/neutron,mattt416/neutron,MaximNevrov/neutron,SamYaple/neutron,sebrandon1/neutron,apporc/neutron,mahak/neutron,cloudbase/neutron,huntxu/neutron,wenhuizhang/neutron,eonpatapon/neutron,yanheven/neutron,dims/neutron,wenhuizhang/neutron,paninetworks/neutron,mandeepdhami/neutron,chitr/neutron,MaximNevrov/neutron,NeCTAR-RC/neutron,cloudbase/neutron,bigswitch/neutron,skyddv/neutron,mattt416/neutron,wolverineav/neutron,skyddv/neutron