Add CLI option to choose model checker engine

test/cmdlineTests/model_checker_engine_none/args

@@ -0,0 +1 @@
+--model-checker-engine none

test/cmdlineTests/model_checker_engine_none/err

@@ -0,0 +1,5 @@
+Warning: SPDX license identifier not provided in source file. Before publishing, consider adding a comment containing "SPDX-License-Identifier: <SPDX-License>" to each source file. Use "SPDX-License-Identifier: UNLICENSED" for non-open-source code. Please see https://spdx.org for more information.
+--> model_checker_engine_none/input.sol
+
+Warning: Source file does not specify required compiler version!
+--> model_checker_engine_none/input.sol

test/cmdlineTests/model_checker_engine_none/input.sol

@@ -0,0 +1,9 @@
+// Removed to yield a warning, otherwise CI test fails with the expectation
+// "no output requested"
+//pragma solidity >=0.0;
+pragma experimental SMTChecker;
+contract test {
+    function f(uint x) public pure {
+		assert(x > 0);
+    }
+}