Removing remaining clippy errors from Bubblegum tests

[danenbm]

• Changes the Result types in tests/utils/mod.rs to use Box<Error> for the error variant, to fix the clippy issue of large enum variant return.
• Removes use of RefCell for Tree and TxBuilder objects. It looks like this was done to allow immutable references to be used for most method calls, and allow for immutable and mutable references to be used at the same time.
  • Overall the use of RefCell was convenient but I don't think it is the best fit for this use case. I'd rather just follow the borrow checker rules. Also, it is not correct to hold a RefCell over await statements, which is fairly fundamental to the test architecture used here.
  • For example, before this code change, a method such as zero_leaf would take an immutable reference to a Tree object instance, and then mutate it because it had a RefCell so it could do a borrow_mut():

  pub fn zero_leaf(&self, index: u32) -> Result<()> {
      let node = [0u8; 32];
      // The conversion below should never fail.
      self.proof_tree
          .borrow_mut()
          .add_leaf(node, usize::try_from(index).unwrap());
      Ok(())
  }

  • Also this allowed taking an immutable reference to a keypair and sending it to a method that would normally require a mutable reference. Thus the borrow checker would not like this either.