Use a new container in each round of push_back test.
Use a new container in each round of push_back test.

Otherwise containers keep growing and uses gigabytes of memory.
nickhutchinson/benchmark,bowlofstew/benchmark,codygray/benchmark,subailong/benchmark,Photonomie/benchmark,disconnect3d/benchmark,google/benchmark,subailong/benchmark,nickhutchinson/benchmark-cxx03,everbase/benchmark,fuchsia-mirror/third_party-benchmark,ntx-/benchmark,gezidan/benchmark,efcs/benchmark,fuchsia-mirror/third_party-benchmark,Photonomie/benchmark,eliben/benchmark,xlqian/benchmark,chemhack/benchmark,mockingbirdnest/benchmark,codygray/benchmark,nickhutchinson/benchmark-cxx03,bowlofstew/benchmark,gezidan/benchmark,codygray/benchmark,xlqian/benchmark,chandlerc/benchmark,disconnect3d/benchmark,everbase/benchmark,google/benchmark,efcs/benchmark,nickhutchinson/benchmark,nickhutchinson/benchmark,ntx-/benchmark,codygray/benchmark,chandlerc/benchmark,quantumsteve/benchmark,nickhutchinson/benchmark-cxx03,chandlerc/benchmark,disconnect3d/benchmark,fuchsia-mirror/third_party-benchmark,everbase/benchmark,chandlerc/benchmark,quantumsteve/benchmark,efcs/benchmark,mockingbirdnest/benchmark,ntx-/benchmark,subailong/benchmark,gezidan/benchmark,bowlofstew/benchmark,nickhutchinson/benchmark-cxx03,quantumsteve/benchmark,fuchsia-mirror/third_party-benchmark,eliben/benchmark,LepelTsmok/benchmark,nickhutchinson/benchmark,LepelTsmok/benchmark,LepelTsmok/benchmark,Photonomie/benchmark,chemhack/benchmark,eliben/benchmark,gezidan/benchmark,chemhack/benchmark,eliben/benchmark,xlqian/benchmark,xlqian/benchmark,quantumsteve/benchmark,mockingbirdnest/benchmark,subailong/benchmark,mockingbirdnest/benchmark,efcs/benchmark,google/benchmark,bowlofstew/benchmark,LepelTsmok/benchmark,disconnect3d/benchmark,everbase/benchmark,chemhack/benchmark,Photonomie/benchmark,ntx-/benchmark