Add testing script for --gen-c-runnable.

[vecchiot-aws]

Description of changes:

Introduces a testing script for --gen-c-runnable which verifies that the generated C code is able to be compiled with gcc.

Resolved issues:

Resolves #445

Call-outs:

This is a draft that I wrote for personal use, but needs to be automated for part of CI.

For this script to work well, the call to CBMC should be disabled to speed up testing and avoid issues with unwinding.

Testing:

• How is this change tested? Existing regression suite.

• Is this a refactor change? No.

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• [] Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.