'test_dbc_parser: ensure we're running on real DBCs'

Split from https://github.com/commaai/opendbc/pull/828

can/tests/test_dbc_parser.py

@@ -6,6 +6,10 @@ from opendbc.can.tests import ALL_DBCS
 
 
 class TestDBCParser(unittest.TestCase):
+  def test_enough_dbcs(self):
+    # sanity check that we're running on the real DBCs
+    self.assertGreater(len(ALL_DBCS), 20)
+
   def test_parse_all_dbcs(self):
     """
       Dynamic DBC parser checks: