feat: add test setup helpers to Lab builder

[Frando]

Description

This adds additive helpers on the lab API to remove boilerplate that every patchbay integration test repeats when it initializes a lab.

Today, iroh's patchbay tests (both iroh and iroh-dns-server) do this to auto-label a lab with the test name:

let mut builder = Lab::builder().outdir(OutDir::Exact(dir.clone()));
if let Some(name) = std::thread::current().name() {
    builder = builder.label(name);
}
let lab = builder.build().await?;
let guard = lab.test_guard();

The if let mutable rebind exists only because label() takes a value while the thread name is an Option. It gets copy-pasted across test files.

This PR adds:

• LabBuilder::label_from_thread() — applies the current thread's name as the label, skipping unnamed threads and the generic runtime names (main, tokio-runtime-worker) so it is safe to call unconditionally. The Rust test harness names each test thread after the test function under the default current-thread #[tokio::test] runtime, which is where this pays off.
• Lab::for_test(dir) — wraps the whole setup: writes output into dir (OutDir::Exact), labels from the thread name, builds, and returns the lab alongside its TestGuard.

The call site collapses to:

let (lab, guard) = Lab::for_test(testdir!()).await?;

or, when a test wants to keep control over the builder:

let lab = Lab::builder()
    .outdir(OutDir::Exact(dir))
    .label_from_thread()
    .build()
    .await?;

Breaking changes

None, only additions