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/1020_bufferoffset_invalid.c

@@ -111,7 +111,7 @@ int T1020_bufferoffset_invalid(const char *initiator, const char *url, int data_
 
 	if (!data_loss) {
 		printf("--dataloss flag is not set. Skipping test\n");
-		ret = -1;
+		ret = -2;
 		goto finished;
 	}