ENH: Use gtest_discover_tests if available
ENH: Use gtest_discover_tests if available

This method is new with CMake 3.10, it runs the executable to
determine the gtests available. This removes the need to re-run CMake
when the tests are modified to determine the tests implemented.

Change-Id: If085aa8c3a547c870ddf8e870bf46eaecada53c2

richardbeare/ITK,richardbeare/ITK,vfonov/ITK,richardbeare/ITK,blowekamp/ITK,LucasGandel/ITK,blowekamp/ITK,richardbeare/ITK,blowekamp/ITK,thewtex/ITK,hjmjohnson/ITK,BRAINSia/ITK,Kitware/ITK,fbudin69500/ITK,malaterre/ITK,stnava/ITK,vfonov/ITK,Kitware/ITK,thewtex/ITK,InsightSoftwareConsortium/ITK,fbudin69500/ITK,stnava/ITK,hjmjohnson/ITK,BRAINSia/ITK,malaterre/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,malaterre/ITK,spinicist/ITK,malaterre/ITK,vfonov/ITK,stnava/ITK,LucasGandel/ITK,malaterre/ITK,hjmjohnson/ITK,spinicist/ITK,BRAINSia/ITK,thewtex/ITK,malaterre/ITK,richardbeare/ITK,vfonov/ITK,spinicist/ITK,thewtex/ITK,fbudin69500/ITK,BRAINSia/ITK,hjmjohnson/ITK,stnava/ITK,stnava/ITK,Kitware/ITK,hjmjohnson/ITK,spinicist/ITK,LucasGandel/ITK,stnava/ITK,fbudin69500/ITK,stnava/ITK,spinicist/ITK,vfonov/ITK,Kitware/ITK,BRAINSia/ITK,hjmjohnson/ITK,stnava/ITK,vfonov/ITK,blowekamp/ITK,BRAINSia/ITK,InsightSoftwareConsortium/ITK,InsightSoftwareConsortium/ITK,blowekamp/ITK,fbudin69500/ITK,Kitware/ITK,malaterre/ITK,LucasGandel/ITK,LucasGandel/ITK,InsightSoftwareConsortium/ITK,richardbeare/ITK,thewtex/ITK,spinicist/ITK,fbudin69500/ITK,richardbeare/ITK,spinicist/ITK,Kitware/ITK,Kitware/ITK,spinicist/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,blowekamp/ITK,malaterre/ITK,spinicist/ITK,stnava/ITK,blowekamp/ITK,malaterre/ITK,thewtex/ITK,fbudin69500/ITK,LucasGandel/ITK,LucasGandel/ITK,thewtex/ITK,fbudin69500/ITK,LucasGandel/ITK,BRAINSia/ITK,blowekamp/ITK