Casper contract not required in tests crate

Inclusion of casper-contract does not seem required in tests

```
casper-engine-test-support = { version = "7.0.1", features = ["test-support"] }
casper-execution-engine = "7.0.1"
casper-types = "4.0.1"
```

https://github.com/casper-ecosystem/cargo-casper/blob/dev/src/tests_package.rs#L25

This leads to conflict for users taking this template as start.