Fix unsat core results

[martinjonas]

The validation of UNSAT core results was not implemented according to the rules. According to the rules

The unsatisfiability of an unsat core is refuted if the number of validation solvers whose result is sat exceeds the number of checking solvers whose result is unsat.

whereas we considered the returned core validated if and only if at least one solver returned result unsat. As a result, the unsat cores where all the validation solvers timed out were incorrectly refuted.