SMTChecker: Bring back counterexample checks in regression tests

Since the default is now to ignore the counterexamples when checking
test output, we bring back counterexample checks in tests where the
counterexample is (mostly) deterministic.

test/libsolidity/smtCheckerTests/simple/cex_smoke_test.sol

@@ -0,0 +1,12 @@
+contract C {
+
+    function f() public pure {
+        uint x = 1;
+        assert(x == 2);
+    }
+}
+// ====
+// SMTEngine: all
+// SMTIgnoreCex: yes
+// ----
+// Warning 6328: (73-87): CHC: Assertion violation happens here.\nCounterexample:\n\nx = 1\n\nTransaction trace:\nC.constructor()\nC.f()