tests: Clean the environment before running tests.

As we start reading more sensitive user configuration (like RT credentials),
this fixture makes it simpler to ensure that the tests never read the user's
real configuration, which could lead to problems like passwords in tracebacks.

tests/conftest.py

@@ -0,0 +1,9 @@
+import os
+
+import pytest
+
+from . import testutil
+
+@pytest.fixture(scope='session', autouse=True)
+def clean_environment():
+    os.environ.pop('CONSERVANCY_REPOSITORY', None)

tests/test_config.py

@@ -39,6 +39,5 @@ def test_repository_from_environment():
         assert config.repository_path().parts == ('/', 'usr', 'local')
 
 def test_no_repository():
-    with update_environ(CONSERVANCY_REPOSITORY=None):
-        config = config_mod.Config()
-        assert config.repository_path() is None
+    config = config_mod.Config()
+    assert config.repository_path() is None