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.

Inspect the Boost.Python test infrastructure and locate all uses of boost_assert. Check how the existing tests behave with NDEBUG defined, then determine the replacement's required behavior and update the affected test-base usages. Done means the tests still report failures reliably when assertions are enabled or disabled.

Written by the indexing model from the issue text.