[qttest] break into debugger on test failure

qttest can throw an exception on test failures, but there is no way to break into a debugger automatically. this is a bit inconvenient, as one would configure the debugger to catch exceptions (and it may catch other exceptions that may be flying around).

ideally we'd have functionality like `std::breakpoint` / `std::is_debugger_present`: and qttest would break on failure if the process is being debugged.

possible implementation approaches are linked in the notes of:
https://en.cppreference.com/w/cpp/utility/breakpoint
https://en.cppreference.com/w/cpp/utility/is_debugger_present