iotests: Perform the correct test in 082

In the "amend" section of 082, we perform a single "convert" test
(namely "convert -o help").  That does not make sense, especially
because we have done exactly that "convert" test earlier in 082 already.

Replacing "convert" by "amend" yields an error, which is correct because
there is no point in "amend" having a default format.  The user has to
either specify the format, or give a file for qemu-img to probe.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>