[cert] capitalize python class constants (#5080)

According to PEP8, constants should be written in all capital letters.

This commit capitalizes these constants:
- thread_cert.TestCase.topology
- thread_cert.TestCase.support_ncp

tests/scripts/thread-cert/test_ipv6_source_selection.py

@@ -37,9 +37,9 @@ ROUTER = 2
 
 
 class TestIPv6SourceSelection(thread_cert.TestCase):
-    support_ncp = False
+    SUPPORT_NCP = False
 
-    topology = {
+    TOPOLOGY = {
         LEADER: {
             'mode': 'rsdn',
             'panid': 0xcafe,