fixtures: Introduce fixture for auto-resetting config object

Autouse fixtures in pytest are a powerful way of executing a
fixture for each test. Since Pegleg uses a global in-memory
object for storing config info, this dictionary should be
cleaned up after each test to ensure test idempotency.

The best way (and cleanest) is to use an autouse fixture inside
a conftest.py which ensures that it is used after each and every
unit test [0].

[0] https://docs.pytest.org/en/latest/fixture.html#conftest-py-sharing-fixture-functions

Change-Id: I0c647d3d7547d1bdcd7c289a4c11159376f9f58d

src/bin/pegleg/tests/unit/conftest.py

@@ -0,0 +1,32 @@
+# Copyright 2018 AT&T Intellectual Property.  All other rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import copy
+
+import pytest
+
+from pegleg import config
+"""Fixtures that are applied to all unit tests."""
+
+
+@pytest.fixture(autouse=True)
+def restore_config():
+    """Used for ensuring the original global context is reset in memory
+    following each test execution. Simply import this file to use.
+    """
+    original_global_context = copy.deepcopy(config.GLOBAL_CONTEXT)
+    try:
+        yield
+    finally:
+        config.GLOBAL_CONTEXT = original_global_context

src/bin/pegleg/tests/unit/fixtures.py

@@ -47,9 +47,6 @@ def _gen_document(**kwargs):
 def create_tmp_deployment_files(tmpdir):
     """Fixture that creates a temporary directory structure."""
     sitenames = ['cicd', 'lab']
-    # Used for ensuring the original global context is reset in memory
-    # following each test execution.
-    original_global_context = copy.deepcopy(config.GLOBAL_CONTEXT)
 
     SITE_TEST_STRUCTURE = {
         'directories': {
@@ -154,6 +151,3 @@ schema: pegleg/SiteDefinition/v1
         files._create_tree(cicd_path, tree=test_structure)
 
     yield
-
-    # Restore the global context back to blank slate status.
-    config.GLOBAL_CONTEXT = original_global_context