[RPC] Improve integration tests with the RPC layer

## Task summary

- Actually the majority of the RPC tests inside `test/rpc` folder are beign skipped or expect and empty data
- Spliting the integration tests by modules
- [x] [Author module](https://github.com/ChainSafe/gossamer/issues/2172)
- [ ] [Chain module](https://github.com/ChainSafe/gossamer/issues/2173)
- [ ] [ChildState module](https://github.com/ChainSafe/gossamer/issues/2174)
- [ ] [Dev module](https://github.com/ChainSafe/gossamer/issues/2175)
- [ ] [GRANDPA module](https://github.com/ChainSafe/gossamer/issues/2176)
- [ ] [Offchain module](https://github.com/ChainSafe/gossamer/issues/2177)
- [ ] [Payment module](https://github.com/ChainSafe/gossamer/issues/2178)
- [ ] [State module](https://github.com/ChainSafe/gossamer/issues/2179)
- [ ] [SyncState module](https://github.com/ChainSafe/gossamer/issues/2180)
- [ ] [System module](https://github.com/ChainSafe/gossamer/issues/2181)

## Specification reference

Would be nice to:
- Rename the files to the module (eg. `system_test.go`)
- Update the `expect` field for every test case, or create a `Test_` function for more complex cases
- Guarantee that the JSON response format is correct

## Other information and links

- N/A