Fix sp-api handling of multiple arguments (#6484)

With the switch to `decode_all_with_depth_limit` we silently broken
support for functions with multiple arguments. The old generated code
tried to decode each parameter separately, which does not play well with
`decode_all`.

This pr adds a test to ensure that this does not happen again and fixes
the bug by decoding everything at once by wrapping it into tuples.

substrate/test-utils/runtime/src/lib.rs

@@ -313,6 +313,9 @@ cfg_if! {
 				fn test_ecdsa_crypto() -> (ecdsa::AppSignature, ecdsa::AppPublic);
 				/// Run various tests against storage.
 				fn test_storage();
+				/// Test that ensures that we can call a function that takes multiple
+				/// arguments.
+				fn test_multiple_arguments(data: Vec<u8>, other: Vec<u8>, num: u32);
 			}
 		}
 	} else {
@@ -359,6 +362,9 @@ cfg_if! {
 				fn test_ecdsa_crypto() -> (ecdsa::AppSignature, ecdsa::AppPublic);
 				/// Run various tests against storage.
 				fn test_storage();
+				/// Test that ensures that we can call a function that takes multiple
+				/// arguments.
+				fn test_multiple_arguments(data: Vec<u8>, other: Vec<u8>, num: u32);
 			}
 		}
 	}
@@ -641,6 +647,11 @@ cfg_if! {
 					test_read_storage();
 					test_read_child_storage();
 				}
+
+				fn test_multiple_arguments(data: Vec<u8>, other: Vec<u8>, num: u32) {
+					assert_eq!(&data[..], &other[..]);
+					assert_eq!(data.len(), num as usize);
+				}
 			}
 
 			impl sp_consensus_aura::AuraApi<Block, AuraId> for Runtime {
@@ -862,6 +873,11 @@ cfg_if! {
 					test_read_storage();
 					test_read_child_storage();
 				}
+
+				fn test_multiple_arguments(data: Vec<u8>, other: Vec<u8>, num: u32) {
+					assert_eq!(&data[..], &other[..]);
+					assert_eq!(data.len(), num as usize);
+				}
 			}
 
 			impl sp_consensus_aura::AuraApi<Block, AuraId> for Runtime {