feat(libp2p-swarm-test): Support tokio

[ark0f]

Description

Notes & open questions

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]

Hi, and thanks for this! Features should be additive, so if we wanted to support both executors we should namespace them. But following #4449 I'd say we can just replace async-std for tokio, wdyt?
cc @dariusc93 @guillaumemichel

[guillaumemichel]

I agree with @jxs, IMO async-std should be replaced by tokio.

[dariusc93]

I do agree with the idea of replacing async-std with tokio as well. Maybe start with the tests and following #4449 we can move on to preferring tokio in other parts of rust-libp2p.