Ensure we spawn the block import worker as an essential task (#8155)

* Ensure we spawn the block import worker as an essential task

This pr ensures that we spawn the block import worker as an essential
task. This is quite important as we need to bring down the node when the
block import is done. Besides that it adds some debug output to the
block import worker.

* Don't be stupid :D

substrate/primitives/core/src/testing.rs

@@ -152,3 +152,13 @@ impl crate::traits::SpawnNamed for TaskExecutor {
 		self.0.spawn_ok(future);
 	}
 }
+
+#[cfg(feature = "std")]
+impl crate::traits::SpawnEssentialNamed for TaskExecutor {
+	fn spawn_essential_blocking(&self, _: &'static str, future: futures::future::BoxFuture<'static, ()>) {
+		self.0.spawn_ok(future);
+	}
+	fn spawn_essential(&self, _: &'static str, future: futures::future::BoxFuture<'static, ()>) {
+		self.0.spawn_ok(future);
+	}
+}