Fix dumping of get-unsat-assumptions (#2302)

Fix dumping of get-unsat-assumptions (#2302)

When dumping a `get-unsat-assumptions` command, CVC4 was instead dumping
two `get-unsat-cores` commands. This commit splits
`SmtEngine::getUnsatCores()` into a part that does the dumping and an
internal part that actually gets the unsat core without dumping.
`SmtEngine::getUnsatAssumptions()` now calls the internal version to
avoid the redundant dumping of a `get-unsat-cores` command and changes
the command that gets dumped in `SmtEngine::getUnsatAssumptions()`.