Check for free variables in several SolverEngine calls (#8130)

Fixes the nightlies due to ensuring that a unit test fails in production (it was failing only in debug).

Fixes #8127.

examples/api/java/HelloWorld.java

@@ -21,7 +21,7 @@ public class HelloWorld
   {
     try (Solver slv = new Solver())
     {
-      Term helloworld = slv.mkVar(slv.getBooleanSort(), "Hello World!");
+      Term helloworld = slv.mkConst(slv.getBooleanSort(), "Hello World!");
 
       System.out.println(helloworld + " is " + slv.checkEntailed(helloworld));
     }