Fixup of escrow clearing after withdrawing, added tests to check

[od-hunter]

Resolves #226

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[ptisserand]

With starknet-foundry 0.19.0, you can directly query storage with the following:

    use snforge_std::{load, map_entry_address};

    let escrowed = load(bridge_address, map_entry_address(selector!("escrow"), array![erc721b_address.into(), 0, 0].span()), 1);

For details see https://book.cairo-lang.org/ch14-01-01-storage-mappings.html and https://docs.starknet.io/architecture-and-concepts/cryptography/hash-functions/#array_hashing

[od-hunter]

@ptisserand Just did a fix , tests are passing on my end.

[od-hunter]

@ptisserand , thanks for the opportunity ser🫡