tests: Use STRNEQ_NULLABLE
tests: Use STRNEQ_NULLABLE

It's possible that the @outbuf and/or @errbuf could be NULL
and thus we need to use the right comparison macro.

Found by Coverity

Signed-off-by: John Ferlan <87558058f6f829e5ec976c8ef960720af4ff9c7d@redhat.com>
ACKed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

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