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_flags.c

@@ -41,6 +41,11 @@ test_writeverify16_flags(void)
 	ret = writeverify16(iscsic, tgt_lun, 0,
 		     block_size, block_size,
 		     0, 1, 0, 0, buf);
+	if (ret == -2) {
+		logging(LOG_NORMAL, "[SKIPPED] WRITE1VERIFY16 is not implemented.");
+		CU_PASS("WRITEVERIFY16 is not implemented.");
+		return;
+       	}	
 	CU_ASSERT_EQUAL(ret, 0);
 
 	logging(LOG_VERBOSE, "Test WRITEVERIFY16 with BYTCHK==1");