[tests] skip running expected fail tests (#5038)

This commit skips running expected fail tests so that it will be easier
to find real failures and saves some time.

tests/scripts/thread-cert/test_coaps.py

@@ -37,6 +37,8 @@ ROUTER = 2
 
 
 class TestCoaps(thread_cert.TestCase):
+    support_ncp = False
+
     topology = {
         LEADER: {
             'mode': 'rsdn',