Activate Z3 if no other SMT solver is configured #3048

Tests

• ✅ key.core.testgen:test (13 succeeded)
• ✅ key.core:test (2919 succeeded, 104 skipped)
• ✅ key.core:testProveRules (196 succeeded)
• ✅ key.core:testRunAllFunProofs (47 succeeded)
• ✅ key.core:testRunAllInfProofs (30 succeeded)
• ✅ key.removegenerics:test (45 succeeded)
• ✅ key.ui:test (5 succeeded)
• ✅ key.util:test (158 succeeded)
• ✅ keyext.exploration:test (3 succeeded)
• ✅ recoder:test (14 succeeded, 15 skipped)