Find way to cleanly run tests on Rustproof

Currently hacked version. Rust must be version 1.12.nightly-2016-08-11 to run tests in current implementation.

With newer version of rust, tests hang due to .git file locks.

Specifically, .git files can only be imported once. Our current system tests spawn new processes using std::process::Command to run Cargo a second time on test files. The first time Cargo is run, .git files are locked.