Rename libcnb-cargo integration test file (#645)

For consistency with all of the other integration tests in this repo:

```
examples/execd/tests/integration_test.rs
examples/ruby-sample/tests/integration_test.rs
test-buildpacks/readonly-layer-files/tests/integration_test.rs
test-buildpacks/sbom/tests/integration_test.rs
test-buildpacks/store/tests/integration_test.rs
libcnb-test/tests/integration_test.rs
```

And also with the naming that the Rust docs use:
https://doc.rust-lang.org/book/ch11-03-test-organization.html#integration-tests