YANG: Prepare for distributing YANG modules

I'm using a second-level module namespace (gnpy.yang) for the same
reasons as when shipping the example data via gnpy/example-data/ --
these will be used in the subsequent commits when we actually start
adding YANG models. There is also some code (not much now, a lot more in
future) for working with these models, and in future also for loading
actual data. These *could* be put into gnpy.tools.*, but I think it's
more straightforward to just keep them in the YANG namespace.

Change-Id: Ic40738ddd8346429bde01e591d19fd2ce8cb687d

tests/test_yang_validation.py

@@ -0,0 +1,46 @@
+# SPDX-License-Identifier: BSD-3-Clause
+#
+# Tests for YANG models of GNPy
+#
+# Copyright (C) 2020 Telecom Infra Project and GNPy contributors
+# see LICENSE.md for a list of contributors
+#
+
+from gnpy.yang import external_path, model_path
+from pathlib import Path
+from typing import List
+import pytest
+import subprocess
+
+
+def _get_basename(filename: Path) -> str:
+    try:
+        return filename.name
+    except AttributeError:
+        return filename
+
+
+@pytest.mark.parametrize("yang_model", external_path().glob('*.yang'), ids=_get_basename)
+def test_lint_external_yang(yang_model):
+    '''Run a basic linter on all third-party models'''
+    _validate_yang_model(yang_model, [])
+
+
+@pytest.mark.parametrize("yang_model", model_path().glob('*.yang'), ids=_get_basename)
+def test_lint_gnpy_yang(yang_model):
+    '''Run a linter on GNPy's YANG models'''
+    _validate_yang_model(yang_model, ('--canonical', '--strict', '--lint'))
+
+
+def _validate_yang_model(filename: Path, options: List[str]):
+    '''Run actual validation'''
+    # I would have loved to use pyang programatically from here, but it seems that the API is really designed
+    # around that interactive use case where code just expects an OptParser as a part of the library context,
+    # etc.
+    # Given that I'm only interested in a simple pass/fail scenario, let's just invoke the linter as a standalone
+    # process and check if it screams.
+    proc = subprocess.run(
+        ('pyang', '-p', ':'.join((str(external_path()), str(model_path()))), *options, filename),
+        stdout=subprocess.PIPE, stderr=subprocess.PIPE, check=True, universal_newlines=True)
+    assert proc.stderr == ''
+    assert proc.stdout == ''