[sonic-yang-models]: fix unit test failure (sonic-net#7436)

https://github.com/mbj4668/pyang/blob/master/pyang/repository.py#L93 throws an exception with pip 21.1

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))