test: Make bootstd init run only on sandbox

Tests for standard boot need disks to be set up, which can only be done
on sandbox, since adjusting disks on real hardware is not currently
supported. Mark the init function as sandbox-only.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/py/tests/test_ut.py b/test/py/tests/test_ut.py
index c169c83..5820506 100644
--- a/test/py/tests/test_ut.py
+++ b/test/py/tests/test_ut.py
@@ -470,6 +470,7 @@
         fh.write(data)
 
 @pytest.mark.buildconfigspec('cmd_bootflow')
+@pytest.mark.buildconfigspec('sandbox')
 def test_ut_dm_init_bootstd(u_boot_console):
     """Initialise data for bootflow tests"""