Fix a check for yang validation (#15498)

[Sonic-Config-Engine] Re-add the yang validation check accidently removed by #13409

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

@@ -57,6 +57,7 @@ class TestCfgGen(TestCase):
 
     def run_script(self, argument, check_stderr=False, verbose=False):
         print('\n    Running sonic-cfggen ' + ' '.join(argument))
+        self.assertTrue(self.yang.validate(argument))
         if check_stderr:
             output = subprocess.check_output(self.script_file + argument, stderr=subprocess.STDOUT)
         else: