tests/isisd: remove auth when fuzzing

tests/isisd: remove auth when fuzzing

Our "deserialize, reserialize, check-equality" test fails when the
fuzzer produces PDUs with incorrect cryptographic checksums.

While the most realistic solution would be to validate the
cryptographic checksums in the test program, that seems very silly,
given that we don't want to fuzz our cryptographic auth.

Given that, removing auth during fuzzing seems the next best solution.

Signed-off-by: Christian Franke <chris@opensourcerouting.org>