Fix MultiSigner, simplify tests (#2033)

* Fix MultiSigner, use `into_signed_tx`

* Rebuild.

substrate/node/runtime/src/lib.rs

@@ -60,7 +60,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	impl_name: create_runtime_str!("substrate-node"),
 	authoring_version: 10,
 	spec_version: 63,
-	impl_version: 65,
+	impl_version: 66,
 	apis: RUNTIME_API_VERSIONS,
 };