ENH: Update CMake Policy settings in Testing/TestExternal project
ENH: Update CMake Policy settings in Testing/TestExternal project

Build tests externally using the same CMake Policy settings as
the top-level of ITK uses.

Change-Id: I333c19317f629fbb7bffbc9a39dbdbed015d4915

zachary-williamson/ITK,InsightSoftwareConsortium/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,blowekamp/ITK,vfonov/ITK,richardbeare/ITK,Kitware/ITK,malaterre/ITK,InsightSoftwareConsortium/ITK,jcfr/ITK,vfonov/ITK,stnava/ITK,Kitware/ITK,jcfr/ITK,spinicist/ITK,BRAINSia/ITK,richardbeare/ITK,LucasGandel/ITK,thewtex/ITK,blowekamp/ITK,PlutoniumHeart/ITK,malaterre/ITK,thewtex/ITK,stnava/ITK,thewtex/ITK,zachary-williamson/ITK,stnava/ITK,fbudin69500/ITK,zachary-williamson/ITK,jcfr/ITK,stnava/ITK,hjmjohnson/ITK,PlutoniumHeart/ITK,BRAINSia/ITK,spinicist/ITK,zachary-williamson/ITK,malaterre/ITK,biotrump/ITK,blowekamp/ITK,malaterre/ITK,malaterre/ITK,blowekamp/ITK,zachary-williamson/ITK,hjmjohnson/ITK,zachary-williamson/ITK,biotrump/ITK,BRAINSia/ITK,vfonov/ITK,fbudin69500/ITK,biotrump/ITK,fbudin69500/ITK,hjmjohnson/ITK,thewtex/ITK,biotrump/ITK,InsightSoftwareConsortium/ITK,zachary-williamson/ITK,LucasGandel/ITK,Kitware/ITK,fbudin69500/ITK,fbudin69500/ITK,InsightSoftwareConsortium/ITK,malaterre/ITK,BRAINSia/ITK,thewtex/ITK,zachary-williamson/ITK,LucasGandel/ITK,biotrump/ITK,Kitware/ITK,hjmjohnson/ITK,stnava/ITK,blowekamp/ITK,jcfr/ITK,biotrump/ITK,zachary-williamson/ITK,BRAINSia/ITK,LucasGandel/ITK,spinicist/ITK,PlutoniumHeart/ITK,richardbeare/ITK,InsightSoftwareConsortium/ITK,fbudin69500/ITK,thewtex/ITK,spinicist/ITK,Kitware/ITK,hjmjohnson/ITK,stnava/ITK,spinicist/ITK,vfonov/ITK,vfonov/ITK,fbudin69500/ITK,LucasGandel/ITK,spinicist/ITK,Kitware/ITK,jcfr/ITK,fbudin69500/ITK,BRAINSia/ITK,blowekamp/ITK,stnava/ITK,vfonov/ITK,PlutoniumHeart/ITK,LucasGandel/ITK,richardbeare/ITK,jcfr/ITK,malaterre/ITK,InsightSoftwareConsortium/ITK,vfonov/ITK,stnava/ITK,spinicist/ITK,jcfr/ITK,spinicist/ITK,stnava/ITK,PlutoniumHeart/ITK,richardbeare/ITK,blowekamp/ITK,spinicist/ITK,PlutoniumHeart/ITK,blowekamp/ITK,vfonov/ITK,PlutoniumHeart/ITK,malaterre/ITK,jcfr/ITK,biotrump/ITK,LucasGandel/ITK,hjmjohnson/ITK,hjmjohnson/ITK,richardbeare/ITK,PlutoniumHeart/ITK,malaterre/ITK,BRAINSia/ITK,biotrump/ITK,LucasGandel/ITK,thewtex/ITK,Kitware/ITK,richardbeare/ITK