chore: swarm executor should match runtime in tests

[elenaf9]

Description

We've changed most of our tests to use the tokio runtime: #4449.
All of these tests, however, were using swarm_test::Swarm::new_ephemeral, which actually creates a Swarm with async_std executor.

This PR changes all test that are using tokio as runtime to use swarm_test::Swarm::new_ephemeral_tokio, so that the executor used by the swarm matches the test's executor.

It also defaults swarm-test to the tokio feature.

Notes & open questions

It also defaults swarm-test to the tokio feature.

Done to prevent that in future PRs and tests Swarm::new_ephemeral is accidentally used again. Crates will need to explicitly enable the async-std feature if they actually want to use that executor.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[jxs]

Thanks for this Elena! LGTM
We could probably also make the Swarm::new_ephemeral use tokio by default in a subsequent PR, wdyt Elena?