Replace `boost_assert` by proper testing logic in the context of BPL tests.

Boost.Python uses calls to `boost_assert` in its testing infrastructure. That macro suffers the same problems as the standard `assert` macro: it may evaluate to a no-op if `NDEBUG` is defined.

We need to find an alternative (which doesn't have to be a macro at all !), and make the corresponding adjustments throughout the Boost.Python test base.