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 via the helper doesn't take longer than a few hundred micro-seconds on my machine.