[sonic-cfggen] Track missing yang models (#10963)

#### Why I did it
sonic-config-engine unit test needs to detect missing yang models.

#### How I did it
Update unit test, return error for missing yang models.

#### How to verify it
Run unit test for sonic-config-engine

src/sonic-config-engine/tests/common_utils.py

@@ -83,6 +83,8 @@ class YangWrapper(object):
             except sonic_yang.SonicYangException as e:
                 print("yang data generated from %s is not valid: %s"%(args.minigraph, str(e)))
                 return False
+            if len(self.yang_parser.tablesWithOutYang):
+                return False
         return True
 
 def cmp(file1, file2):