chore: clean up conductor chain ID tests

[ethanoroshiba]

As indicated in this comment the conductor chain ID check tests are quite messy due to the manual mounting and testing for the correct exit error. We should find a way to clean this up.

astria/crates/astria-conductor/tests/blackbox/firm_only.rs

Lines 417 to 419 in 8510b64

	// We have to create our own test conductor and perform mounts manually because `TestConductor`
	// implements the `Drop` trait, which disallows us from taking ownership of its tasks and
	// awaiting their completion.

astria/crates/astria-conductor/tests/blackbox/soft_only.rs

Lines 371 to 373 in 8510b64

	// We have to create our own test conductor and perform mounts manually because `TestConductor`
	// implements the `Drop` trait, which disallows us from taking ownership of its tasks and
	// awaiting their completion.

┆Issue Number: ENG-892