Compiletest mode for executing arbitrary commands

[tedinski]

We have several ad-hoc bash scripts for testing in Kani, it would be nice if we had a compiletest mode that:

1. Could execute an arbitrary command, and
2. Performed expect test validation on the output.

Given:

• compiletest doesn't check cargo-kani's exit status #1895

This solution should probably check exit status (for a particular value, just default 0) as well.