Allow certain tests to run only on odra-vm.

[zie1ony]

It happens that I want to write submodule without making it into a contract.
What we do is we build mock around it, add to Odra.toml and test it.
I propose different approach: allow to write tests as odra_vm only, that runs against odra_vm also when cargo odra test -b casper.

The easiest way to do it is making fn odra_env() -> HostEnv from odra_test a public method.
This way I could write tests against odra_env.