test: dm: Show failing driver name

When a driver is not registered properly it is not clear which one it
is. Adjust test_dm_compat() to show this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/py/tests/test_dm.py b/test/py/tests/test_dm.py
index 68d4ea1..be94971 100644
--- a/test/py/tests/test_dm.py
+++ b/test/py/tests/test_dm.py
@@ -13,8 +13,11 @@
                for line in response[:-1].split('\n')[2:])
 
     response = u_boot_console.run_command('dm compat')
+    bad_drivers = set()
     for driver in drivers:
-        assert driver in response
+        if not driver in response:
+            bad_drivers.add(driver)
+    assert not bad_drivers
 
     # check sorting - output looks something like this:
     #  testacpi      0  [   ]   testacpi_drv          |-- acpi-test