tests: Fix nbdkit sh script error

The test log for opt-info shows:

nbdkit: sh: error: /home/eblake/libnbd/tests/opt-info.sh: script exited with error, but did not print an error message on stderr

The test happened to pass in spite of the bug in our sh plugin script,
but we are better off reporting errors correctly.

Fixes: 517515e5 ("api: add nbd_opt_info, nbd_aio_opt_info", v1.3.12)