test/py: add skip marker for reliance on tools

Some tests use external tools (executables) during their operation. Add
a test.py mark to indicate this. This allows those tests to be skipped if
the required tool is not present.

Signed-off-by: Stephen Warren <swarren@nvidia.com>

test/py/tests/test_dfu.py

@@ -113,6 +113,7 @@ test_sizes_default = (
 first_usb_dev_port = None
 
 @pytest.mark.buildconfigspec('cmd_dfu')
+@pytest.mark.requiredtool('dfu-util')
 def test_dfu(u_boot_console, env__usb_dev_port, env__dfu_config):
     """Test the "dfu" command; the host system must be able to enumerate a USB
     device when "dfu" is running, various DFU transfers are tested, and the