[sonic-yang-models]: fix unit test failure

it is not quite understood why the unit test suddenly failed as below.
but add ietf yang model explicitly to the build process fix the test
failure.

tests/test_sonic_yang_models.py .F [ 66%]
tests/yang_model_tests/test_yang_model.py . [100%]
Failed: pyang -f tree ./yang-models/*.yang > ./yang-models/sonic_yang_tree
----------------------------- Captured stderr call -----------------------------
./yang-models/sonic-acl.yang:8: error: module "ietf-inet-types" not found in search path
./yang-models/sonic-device_metadata.yang:8: error: module "ietf-yang-types" not found in search path

Signed-off-by: Guohan Lu <[email protected]>

src/sonic-yang-models/tests/test_sonic_yang_models.py

@@ -24,9 +24,12 @@ def test_content(response):
 def test_generate_yang_tree():
 
     # Generate YANG Tree, see no error in it.
-    pyang_tree_cmd = "pyang -f tree ./yang-models/*.yang > ./yang-models/sonic_yang_tree"
+    pyang_tree_cmd = "pyang -Vf tree -p /usr/local/share/yang/modules/ietf ./yang-models/*.yang > ./yang-models/sonic_yang_tree"
     if (system(pyang_tree_cmd)):
         print("Failed: {}".format(pyang_tree_cmd))
+        system("pyang --version")
+        system("env")
+        system("ls -l /usr/local/share/yang/modules/ietf/")
         exit(1)
     else:
         print("Passed: {}".format(pyang_tree_cmd))