fix: scope MutexGuard in worker thread to not hold lock during test

The original code held the work_rx mutex for the entire recv() call.
While harmless when items are pre-buffered, this is incorrect — scope
the guard so it drops after recv() returns but before running the test.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/openwa-test-runner/src/main.rs

@@ -419,14 +419,17 @@ fn run_tests_parallel(
 
         handles.push(thread::spawn(move || {
             loop {
-                // Lock only long enough to recv, then drop the guard before running the test
-                let item = rx.lock().unwrap().recv();
+                let item = {
+                    let guard = rx.lock().unwrap();
+                    guard.recv()
+                    // MutexGuard drops here — lock released before running the test
+                };
                 match item {
                     Ok((idx, test)) => {
                         let result = run_test(&test, &launcher, &wa_exe, &run_dir);
                         let _ = tx.send((idx, result));
                     }
-                    Err(_) => break, // Channel closed, no more work
+                    Err(_) => break,
                 }
             }
         }));