FindGTest: use new add_test() signature (#14913)

This allows to use target names as executables.

Modules/FindGTest.cmake

@@ -134,7 +134,7 @@ function(GTEST_ADD_TESTS executable extra_args)
             message(WARNING "Could not parse GTest ${hit} for adding to CTest.")
             continue()
           endif()
-          add_test(${test_name} ${executable} --gtest_filter=${test_name} ${extra_args})
+          add_test(NAME ${test_name} COMMAND ${executable} --gtest_filter=${test_name} ${extra_args})
         endforeach()
     endforeach()
 endfunction()