Ensure that table router is always built (#952)

* Ensure that table router is always build

This pr ensures that the table router is always build, aka the future is
resolved. This is important, as the table router internally spawns tasks
to handle gossip messages. Handling gossip messages is not only required
on parachain validators, but also on relay chain validators to receive collations.

Tests are added to ensure that the assumptions hold.

* Fix compilation

* Switch to closures

* Remove empty line

* Revert "Remove empty line"

This reverts commit 0d4aaba1780aec1c8d61e1d5dcf7768918af02d9.

* Revert "Switch to closures"

This reverts commit d128c4ecc02c911552a3bfd2142b5a4f7b1338ba.

* Hybrid approach

* Rename test

* Make trait crate local

polkadot/runtime/test-runtime/src/lib.rs

@@ -53,7 +53,7 @@ use frame_support::{
 	weights::DispatchInfo,
 };
 use pallet_transaction_payment_rpc_runtime_api::RuntimeDispatchInfo;
-use session::{historical as session_historical};
+use session::historical as session_historical;
 
 #[cfg(feature = "std")]
 pub use staking::StakerStatus;