refactor: dedup runtime initializations (#22263)

Co-authored-by: Alexey Shekhirin <github@shekhirin.com>

examples/custom-engine-types/src/main.rs

@@ -391,7 +391,7 @@ where
 async fn main() -> eyre::Result<()> {
     let _guard = RethTracer::new().init()?;
 
-    let runtime = Runtime::with_existing_handle(tokio::runtime::Handle::current())?;
+    let runtime = Runtime::test();
 
     // create genesis with canyon at block 2
     let spec = ChainSpec::builder()