test(esplora): speed up test_finalize_chain_update

[ValuedMammal]

The test test_finalize_chain_update was taking up a lot of time during testing. This PR reduces the time to run the tests to around 6 seconds. I stripped down the test logic to the bare essentials. I think it is adequate but let me know if you have other ideas.

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[evanlinjin]

Concept ACK, Approach NACK

IMO the old tests were a bit more thorough with the testing, and also easier to extend with more test cases. For performance, I think the main culprit is the starting and stopping of the test env.

I think it would make sense to make the test cases use relative heights.

struct TestCase {
    /// Initial checkpoint heights, relative to the starting tip height of the env.
    initial_cps: &'a [i32],
    /// Number of blocks mined since starting env tip. The new tip should be introduced by the update.
    chain_extends_by: i32,
    /// Anchors passed into `chain_update`: `(height_relative_to_starting_tip, txid)`.
    anchors: &[(i32, Txid)],
}

Another option is to keep how TestCase is currently, reuse the TestEnv between tests, but make sure the a TestCase's initial_env_height is always equal or greater than the last TestCase's final_env_height. I think I like this solution better.