iscsi-test: return -2 when skipping tests

This lets a test driver distinguish failure from skipping unsupported
tests.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

test-tool/0331_writeverify16_wrprotect.c

@@ -70,13 +70,14 @@ int T0331_writeverify16_wrprotect(const char *initiator, const char *url, int da
 	if(rc16->prot_en != 0) {
 		printf("device is formatted with protection information, skipping test\n");
 		scsi_free_scsi_task(task);
+		ret = -2;
 		goto finished;
 	}
 	scsi_free_scsi_task(task);
 
 	if (!data_loss) {
 		printf("--dataloss flag is not set. Skipping test\n");
-		ret = -1;
+		ret = -2;
 		goto finished;
 	}