test, init: perturb file to ensure failure instead of only deleting them

[brunoerg]

In feature_init.py there is a TODO about perturbing the files instead of only testing by deleting them.

            # TODO: at some point, we should test perturbing the files instead of removing
            # them, e.g.
            #
            # contents = target_file.read_bytes()
            # tweaked_contents = bytearray(contents)
            # tweaked_contents[50:250] = b'1' * 200
            # target_file.write_bytes(bytes(tweaked_contents))
            #
            # At the moment I can't get this to work (bitcoind loads successfully?) so
            # investigate doing this later.

This PR adds it by writing into the file random bytes and checking whether it throws an error when starting.

[DrahtBot]

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[maflcko]

lgtm ACK c371cae

[fanquake]

Note that it looks like this didn't work, and is being fixed in #27823.