test: dedup file hashing using `sha256sum_file` helper

Rather than doing the open/read/hash-steps manually in the affected
functional tests, we can just use the `sha256sum_file` helper from the
utils module instead.

Note that for the tool_wallet.py test, the used hash is changed from
sha1 to sha256, but as the only purpose is to detect file content
changes, this doesn't matter. Also, the optimization using `memoryview`
is overkill here, as the opened file has only a size of 24KiB and
determining the hash doesn't take longer than a few hundred
micro-seconds on my machine.

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