tests: 🤝 hoist test subscriber into a standalone library

[cratelyn]

in #4193, @zbuc is working on some changes to the dex component. in the penumbra-app crate, we have a helpful utility to define a tracing subscriber that correctly interacts with libtest's output capturing, and can be configured via RUST_LOG when running tests.

this is a tremendously helpful aid in debugging issues during feature development, and is not specific to the penumbra-app crate.

this commit hoists that test utility into a standalone library in crates/test/, so that other components like the dex can also create a guard to capture traces in tests.

now we can share this, by doing this:

 #[tokio::test]
 async fn example_test_case() -> anyhow::Result<()> {
     // Install a test logger...
     let guard = set_tracing_subscriber();

     // Test logic here...

     drop(guard);
     Ok(())
 }

checklist before requesting a review

• If this code contains consensus-breaking changes, I have added the "consensus-breaking" label. Otherwise, I declare my belief that there are not consensus-breaking changes, for the following reason:

  only changes test code