bug#29239: FYI: tests: unpack-invalid: correct and clean up a test

[Jim Meyering]

Noticed the problem incidentally, I've just pushed this:

    tests: unpack-invalid: correct and clean up a test
    * tests/unpack-invalid: There was a logic error that would have
    caused this test to ignore a failure if first iteration of the
    loop set fail=1 and the second one reset it to 0.  Also, use
    "returns_ 1 ...", to require an exit status of 1.

Here's the diff:

  https://git.sv.gnu.org/cgit/gzip.git/commit/?id=15bd3ba5d514af6a3