ans_check<T>() should switch to absolute tolerance based floating point comparison for values close to 0

[rigarash]

Currently, we use ans_check() helper function[1] for comparing floating point values using relative tolerance.
However, using relative tolerance does not work with results close to 0.
Therefore, we need to switch from absolute tolerance to relative tolerance seamlessly[2].

[1] https://github.com/ricosjp/monolish/blob/master/test/test_utils.hpp#L20
[2] https://www.boost.org/doc/libs/1_76_0/libs/test/doc/html/boost_test/testing_tools/extended_comparison/floating_point/floating_points_comparison_impl.html