test/crypto: skip plain text compare for null cipher

test/crypto: skip plain text compare for null cipher

NULL cipher is used for validating auth only cases. With NULL cipher,
validating plain text should not be done as the PMD is only expected
to update auth data.

Fixes: e847fc512817 ("test/crypto: add encrypted digest case for AES-CTR-CMAC")
Cc: stable@dpdk.org
Signed-off-by: Anoob Joseph <anoobj@marvell.com>
Acked-by: Ciara Power <ciara.power@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>