Snowbridge - Test pallet order (#3381)

- Adds a test to check the correct digest for Snowbridge outbound
messages. For the correct digest to be in the block, the the
MessageQueue pallet should be configured after the EthereumOutbound
queue pallet. The added test fails if the EthereumOutbound is configured
after the MessageQueue pallet.
- Adds a helper method `run_to_block_with_finalize` to simulate the
block finalizing. The existing `run_to_block` method does not finalize
and so it cannot successfully test this condition.

Closes: https://github.com/paritytech/polkadot-sdk/issues/3208

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Co-authored-by: claravanstaden <Cats 4 life!>