Do not make SyGuS subsolver in unsat state after solving (#7759)

Do not make SyGuS subsolver in unsat state after solving (#7759)

This makes subsolvers answer "unknown" instead of "unsat" when solving SyGuS inputs, by setting unknown when a SyGuS input is solved instead of asserting the negation of the input. Knowing whether the call was successful is simply obtained by calling getSynthSolutions.

This will allow for multiple solutions for the same conjecture.