chore: move block observer testing stuff around

[cylewitruk]

Description

Writing some block observer tests and the file was getting a bit long, so I refactored out the test harness etc. to a separate file in testing like we do for the signer & coordinator.

Pulling this into a separate PR to make review easy.

Changes

95% organizational, the rest is just cargo fmt + clippy + some getters on the test harness since now the fields aren't directly accessible from tests in the "real" block observer file.

Testing Information

Nothing special, everything is as before.

Checklist:

• I have performed a self-review of my code
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[djordon]

Looks good ✅