test: various style improvements to abc-segwit-recovery.py
test: various style improvements to abc-segwit-recovery.py

Summary:
- remove unused functions `check_mempool_equal`
- remove single liner functions `save_spendable_output` (a simple append to a local list) and `get_spendable_output` (only one callsite)
- rename `next_block` to `make_block`, and remove `block` shorthand (`block` is also used a variable name in `update_block`)
- remove the need to keep a dictionary of blocks (with arbitrary magic int as a key), a dictionary of block heights, and a reference to the tip block, by simply adding a `tip_height` counter and passing a block parameter to method `make_block` and function `update_block`.
- use the block returned by `make_block` instead of `self.tip`.

-20 lines of codes with no change in behavior and more clarity.

Depends on D10321

Test Plan: `test/functional/test_runner.py abc-segwit-recovery.py`

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10329

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