[test][length] do not test the variant-related code if GEOMETRY_TEST_DEBUG is defined
(the variant-related code does not compile when GEOMETRY_TEST_DEBUG is defined)
[test][perimeter] do not test the variant-related code if GEOMETRY_TEST_DEBUG is defined
(the variant-related code does not compile when GEOMETRY_TEST_DEBUG is defined);
replace bg::perimeter_result (non-existing) by bg::default_lentgh_result (error
produced when compiling with -DGEOMETRY_TEST_DEBUG)