Remove the magic number `1800` in favor of a generic submission
interval. Based on the test's comment, this is the expected input.

Breaking this into its own test from https://github.com/ethereum-optimism/optimism/pull/7928
to show that without the rest of the diff, the test still passes to
ensure that a bug doesn't sneak in.

fwiw this test will likely be modified with the next release of the
fault proof.