chainHead/tests: Fix clippy (#2325)

Remove the genesis hash from tests:
- Clippy was passing on the PR:
https://github.com/paritytech/polkadot-sdk/pull/2296
- Clippy fails on master:
https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/4328487

This was a race with merging:
https://github.com/paritytech/polkadot-sdk/pull/2295, which introduced
another test that used the `CHAIN_GENESIS`

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>