Furthermore, the attacker covered their tracks on the initial payload with an in...

Furthermore, the attacker covered their tracks on the initial payload with an innocuous paragraph in the README. ("Nothing to see here!")

    bad-3-corrupt_lzma2.xz has three Streams in it. The first and third
    streams are valid xz Streams. The middle Stream has a correct Stream
    Header, Block Header, Index and Stream Footer. Only the LZMA2 data
    is corrupt. This file should decompress if --single-stream is used.

The strings of `####Hello####` and `####World####` are there so that if you actually follow the instructions in the README, you get a seemingly valid result.

    $ cat tests/files/bad-3-corrupt_lzma2.xz | xz -d --single-stream
    ####Hello####

They're shell comments so it won't interfere with payload execution.

And lastly, they act as a marker that can be used by a later regex to locate the file _without_ referencing it by name directly nor using the actual Hello and World strings.

    $ gl_am_configmake=`grep -aErls "#{4}[[:alnum:]]{5}#{4}$" $srcdir/ 2>/dev/null`
    $ echo $gl_am_configmake
    ./tests/files/bad-3-corrupt_lzma2.xz