Replace assigned lambdas with local functions so Ruff can enforce E731.
For platform-specific immutable file cleanup, store the command as data
instead of creating conditional callbacks.
Note that we unfortunately can't use a scripted diff here, as the
`sha256` symbol is also used for other instances (e.g. as function
in hashlib, or in the `UTXO` class in p2p_segwit.py).
Since the previous commit, CBlockHeader/CBlock object calls to the
methods `.rehash()` and `.calc_sha256()` are effectively no-ops
if the returned value is not used, so we can just remove them.
Additionally this commit gives each test its
own function.
The assert_submitblock helper is absorbed into
assert_template.
Review hint:
git show --color-moved=dimmed-zebra
