Prevent crash in helper.py when program outputs null bytes

[XavierCooney]

If the program being tested outputs a null byte to stdout autotest crashes with

Traceback (most recent call last):
  File "/usr/local/share/autotest/autotest.py", line 41, in main
    sys.exit(run_autotest())
             ^^^^^^^^^^^^^^
  File "/usr/local/share/autotest/autotest.py", line 111, in run_autotest
    run_helper(tests, parameters, args)
  File "/usr/local/extrafiles/share/autotest/helper.py", line 56, in run_helper
    os.environ["HELPER_" + k.upper()] = v
    ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen os>", line 685, in __setitem__
ValueError: embedded null byte

This is because the stdout of the program is passed to the helper via an environment variable, which does not permit embedding null bytes. Similar issues occur with the stdin/stderr/expected_stdout/source.

This PR replaces the null bytes with \x00, which I assume the LLM can still make sense of.