Graduate these tests to unit tests

Although they don't use Boost::UnitTest, they can at least pass, so we
can use them to detect simple compilation and runtime errors, although
we miss the semantic check.

(If you have time, please make them proper unit tests)

CMakeLists_files.cmake

@@ -137,6 +137,7 @@ list (APPEND MAIN_SOURCE_FILES
 # find tests -name '*.cpp' -a ! -wholename '*/not-unit/*' -printf '\t%p\n' | sort
 list (APPEND TEST_SOURCE_FILES
 	tests/test_dgbasis.cpp
+	tests/test_cubic.cpp
 	tests/test_nonuniformtablelinear.cpp
 	tests/test_sparsevector.cpp
 	tests/test_sparsetable.cpp