Add integration tests

[ForestEckhardt]

We should add integration tests that test the overall functionality of the rust buildpack.

[dmikusa]

This is probably where I see the integration tests best fitting. I have some fixtures, not committed that we can use. It should be fairly straightforward to cover the use cases for these CNBs with tests.

Trying to learn from the fun I've had with the PHP buildpacks, I want to be careful about the fixtures we take on so that we can make sure they are maintainable.