TESTS: WRITEVERIFY10/12/16 are all optional.

Update the helpers to return -2/not supported on invalid operaction code.
Update all tests to accept that the opcode might be missing and abort the test without failing.

test-tool/test_writeverify16_simple.c

@@ -42,6 +42,11 @@ test_writeverify16_simple(void)
 		ret = writeverify16(iscsic, tgt_lun, 0, i * block_size,
 		    block_size, 0, 0, 0, 0, buf);
 		free(buf);
+		if (ret == -2) {
+			logging(LOG_NORMAL, "[SKIPPED] WRITE1VERIFY16 is not implemented.");
+			CU_PASS("WRITEVERIFY16 is not implemented.");
+			return;
+	       	}	
 		CU_ASSERT_EQUAL(ret, 0);
 	}