Fix errors due to non-explicit conversions of tribool to bool

[DenizThatMenace]

This fixes errors like the following that occur with newer compiler versions:

/usr/include/boost/test/tools/assertion_result.hpp:57:79: error: no matching function for call to ‘boost::test_tools::assertion_result::readonly_property65::readonly_property65(boost::logic::tribool)’
   57 |     assertion_result( BoolConvertable const& pv_ ) : p_predicate_value( !!pv_ ) {}
      |                                                                               ^