[author-inherent] Move kick_off_authorship_validation() to post_inherents()

[arturgontijo]

This PR moves the kick_off_authorship_validation() logic to the post_inherents().

It also adds integrity_test() hook with some checks.

Note: Do we need more integrity checks there? I decided to keep tests.rs simple as this new approach is.

As we see here, integrity_tests is also being tested too.

➜  moonkit git:(artur/post-inherents) cargo test -p pallet-author-inherent                                                        
warning: profile package spec `cranelift-wasm` in profile `dev` did not match any packages
    Finished `test` profile [unoptimized + debuginfo] target(s) in 1.27s
     Running unittests src/lib.rs (target/debug/deps/pallet_author_inherent-95dea1c55ff74551)

running 3 tests
test mock::__construct_runtime_integrity_test::runtime_integrity_tests ... ok
test mock::test_genesis_config_builds ... ok
test tests::test_author_is_extracted_and_stored_from_pre_runtime_digest ... ok

test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.01s