sp-api: Support nested transactions (#14447)

* sp-api: Support nested transactions

Adds support for nested transactions in `sp-api` by using `execute_in_transaction`. This was working
until a recent refactor, but this was actually not intended. However, supporting nested transactions
is a worthwhile feature to have. So, this pr "brings it back" and adds a test to ensure it will not break.

* Make clippy happy

* Assert that the runtime api type is not unwind safe

* Count number of transactions

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

@@ -217,6 +217,8 @@ decl_runtime_apis! {
 		fn do_trace_log();
 		/// Verify the given signature, public & message bundle.
 		fn verify_ed25519(sig: ed25519::Signature, public: ed25519::Public, message: Vec<u8>) -> bool;
+		/// Write the given `value` under the given `key` into the storage and then optional panic.
+		fn write_key_value(key: Vec<u8>, value: Vec<u8>, panic: bool);
 	}
 }
 
@@ -606,6 +608,14 @@ impl_runtime_apis! {
 		fn verify_ed25519(sig: ed25519::Signature, public: ed25519::Public, message: Vec<u8>) -> bool {
 			sp_io::crypto::ed25519_verify(&sig, &message, &public)
 		}
+
+		fn write_key_value(key: Vec<u8>, value: Vec<u8>, panic: bool) {
+			sp_io::storage::set(&key, &value);
+
+			if panic {
+				panic!("I'm just following my master");
+			}
+		}
 	}
 
 	impl sp_consensus_aura::AuraApi<Block, AuraId> for Runtime {