support/testing: fix MyPy warnings about BRConfigTest

This removes warnings in editors/IDEs with MyPy typechecking
integration. Test classes override "config" with strings (different
type than None).

Signed-off-by: Fiona Klute (WIWA) <fiona.klute@gmx.de>
Signed-off-by: Julien Olivain <ju.o@free.fr>

support/testing/infra/basetest.py

@@ -24,8 +24,8 @@ MINIMAL_CONFIG = \
 
 class BRConfigTest(unittest.TestCase):
     """Test up to the configure stage."""
-    config = None
-    br2_external = list()
+    config: str
+    br2_external: list[str] = list()
     downloaddir = None
     outputdir = None
     logtofile = True