chore: separate sequencer testing only utils and utils shared with benchmark feature

[ethanoroshiba]

We should figure out a way to get rid of this and the others in this file. Unfortunately, we cannot replace allow inside cfg_attr with expect, since clippy evaluates it even when the benchmark feature is not enabled and errs with unfulfilled_lint_expectations.

astria/crates/astria-sequencer/src/app/test_utils.rs

Lines 59 to 65 in e267d1c

	#[expect(
	clippy::allow_attributes,
	clippy::allow_attributes_without_reason,
	reason = "allow is only necessary when benchmark isn't enabled"
	)]
	#[cfg_attr(feature = "benchmark", allow(dead_code))]
	pub(crate) fn get_alice_signing_key() -> SigningKey {

┆Issue Number: ENG-877