Fix warnings in registration/test_registration_api.cpp
Fix warnings in registration/test_registration_api.cpp

> test_registration_api.cpp:414:18: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
> test_registration_api.cpp:457:18: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
> test_registration_api.cpp:552:21: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
> test_registration_api.cpp:590:21: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]

sbec/pcl,sbec/pcl,3dtof/pcl,locnx1984/pcl,shyamalschandra/pcl,LZRS/pcl,jeppewalther/kinfu_segmentation,zavataafnan/pcl-truck,closerbibi/pcl,cascheberg/pcl,fanxiaochen/mypcltest,closerbibi/pcl,stfuchs/pcl,sbec/pcl,zavataafnan/pcl-truck,ipa-rmb/pcl,Tabjones/pcl,lydhr/pcl,cascheberg/pcl,mschoeler/pcl,locnx1984/pcl,fanxiaochen/mypcltest,chenxingzhe/pcl,simonleonard/pcl,3dtof/pcl,pkuhto/pcl,shivmalhotra/pcl,raydtang/pcl,v4hn/pcl,kanster/pcl,starius/pcl,the-glu/pcl,lydhr/pcl,starius/pcl,soulsheng/pcl,v4hn/pcl,kanster/pcl,LZRS/pcl,starius/pcl,lydhr/pcl,LZRS/pcl,MMiknis/pcl,stfuchs/pcl,starius/pcl,stfuchs/pcl,zhangxaochen/pcl,krips89/pcl_newfeatures,the-glu/pcl,jeppewalther/kinfu_segmentation,lebronzhang/pcl,msalvato/pcl_kinfu_highres,nikste/pcl,jeppewalther/kinfu_segmentation,stefanbuettner/pcl,RufaelDev/pcc-mp3dg,ResByte/pcl,damienjadeduff/pcl,Tabjones/pcl,KevenRing/vlp,krips89/pcl_newfeatures,shivmalhotra/pcl,KevenRing/vlp,cascheberg/pcl,raydtang/pcl,stfuchs/pcl,damienjadeduff/pcl,sbec/pcl,LZRS/pcl,nikste/pcl,LZRS/pcl,jakobwilm/pcl,mschoeler/pcl,closerbibi/pcl,jakobwilm/pcl,shyamalschandra/pcl,shangwuhencc/pcl,lebronzhang/pcl,jeppewalther/kinfu_segmentation,soulsheng/pcl,Tabjones/pcl,lebronzhang/pcl,the-glu/pcl,fskuka/pcl,MMiknis/pcl,RufaelDev/pcc-mp3dg,jakobwilm/pcl,wgapl/pcl,KevenRing/vlp,drmateo/pcl,nikste/pcl,mikhail-matrosov/pcl,jakobwilm/pcl,drmateo/pcl,v4hn/pcl,kanster/pcl,soulsheng/pcl,MMiknis/pcl,wgapl/pcl,shivmalhotra/pcl,zhangxaochen/pcl,fskuka/pcl,shangwuhencc/pcl,shyamalschandra/pcl,krips89/pcl_newfeatures,nikste/pcl,v4hn/pcl,MMiknis/pcl,msalvato/pcl_kinfu_highres,lebronzhang/pcl,DaikiMaekawa/pcl,zavataafnan/pcl-truck,damienjadeduff/pcl,KevenRing/pcl,ipa-rmb/pcl,stfuchs/pcl,fskuka/pcl,srbhprajapati/pcl,cascheberg/pcl,Tabjones/pcl,locnx1984/pcl,cascheberg/pcl,drmateo/pcl,ipa-rmb/pcl,chatchavan/pcl,pkuhto/pcl,DaikiMaekawa/pcl,msalvato/pcl_kinfu_highres,zavataafnan/pcl-truck,chatchavan/pcl,3dtof/pcl,stefanbuettner/pcl,stefanbuettner/pcl,the-glu/pcl,ResByte/pcl,shyamalschandra/pcl,shangwuhencc/pcl,soulsheng/pcl,raydtang/pcl,fanxiaochen/mypcltest,chenxingzhe/pcl,KevenRing/pcl,shangwuhencc/pcl,zavataafnan/pcl-truck,fanxiaochen/mypcltest,stefanbuettner/pcl,simonleonard/pcl,kanster/pcl,shivmalhotra/pcl,MMiknis/pcl,ipa-rmb/pcl,raydtang/pcl,krips89/pcl_newfeatures,chatchavan/pcl,3dtof/pcl,pkuhto/pcl,mschoeler/pcl,DaikiMaekawa/pcl,kanster/pcl,locnx1984/pcl,wgapl/pcl,srbhprajapati/pcl,ResByte/pcl,closerbibi/pcl,lydhr/pcl,chenxingzhe/pcl,fanxiaochen/mypcltest,shangwuhencc/pcl,lebronzhang/pcl,chatchavan/pcl,mikhail-matrosov/pcl,KevenRing/pcl,mikhail-matrosov/pcl,simonleonard/pcl,simonleonard/pcl,zhangxaochen/pcl,srbhprajapati/pcl,fskuka/pcl,zhangxaochen/pcl,chenxingzhe/pcl,closerbibi/pcl,KevenRing/pcl,nikste/pcl,mschoeler/pcl,DaikiMaekawa/pcl,mschoeler/pcl,KevenRing/vlp,fskuka/pcl,srbhprajapati/pcl,msalvato/pcl_kinfu_highres,ResByte/pcl,zhangxaochen/pcl,simonleonard/pcl,RufaelDev/pcc-mp3dg,damienjadeduff/pcl,soulsheng/pcl,KevenRing/pcl,DaikiMaekawa/pcl,damienjadeduff/pcl,jakobwilm/pcl,starius/pcl,RufaelDev/pcc-mp3dg,mikhail-matrosov/pcl,pkuhto/pcl,lydhr/pcl,srbhprajapati/pcl,v4hn/pcl,the-glu/pcl,drmateo/pcl,pkuhto/pcl,Tabjones/pcl,ResByte/pcl,chenxingzhe/pcl,sbec/pcl,krips89/pcl_newfeatures,ipa-rmb/pcl,wgapl/pcl,KevenRing/vlp,raydtang/pcl,drmateo/pcl,RufaelDev/pcc-mp3dg,locnx1984/pcl,jeppewalther/kinfu_segmentation,wgapl/pcl,shivmalhotra/pcl,msalvato/pcl_kinfu_highres,stefanbuettner/pcl,mikhail-matrosov/pcl,3dtof/pcl,shyamalschandra/pcl