Make PanBase hermetic across tests by clearing _config each time (#249)

* Make PanBase hermetic across tests by clearing _config each time.

pocs/base.py

@@ -10,6 +10,16 @@
 _config = None
 
 
+def reset_global_config():
+    """Reset the global _config to None.
+
+    Globals such as _config make tests non-hermetic. Enable conftest.py to clear _config
+    in an explicit fashion.
+    """
+    global _config
+    _config = None
+
+
 class PanBase(object):
 
     """ Base class for other classes within the PANOPTES ecosystem

pocs/tests/conftest.py

@@ -1,10 +1,15 @@
+import copy
 import os
 import pytest
 
+import pocs.base
 from pocs import hardware
 from pocs.utils.config import load_config
 from pocs.utils.database import PanMongo
 
+# Global variable with the default config; we read it once, copy it each time it is needed.
+_one_time_config = None
+
 
 def pytest_addoption(parser):
     parser.addoption("--with-hardware", nargs='+', default=[],
@@ -44,11 +49,16 @@ def pytest_collection_modifyitems(config, items):
                 item.add_marker(skip)
 
 
-@pytest.fixture
+@pytest.fixture(scope='function')
 def config():
-    config = load_config(ignore_local=True, simulator=['all'])
-    config['db']['name'] = 'panoptes_testing'
-    return config
+    pocs.base.reset_global_config()
+
+    global _one_time_config
+    if not _one_time_config:
+        _one_time_config = load_config(ignore_local=True, simulator=['all'])
+        _one_time_config['db']['name'] = 'panoptes_testing'
+
+    return copy.deepcopy(_one_time_config)
 
 
 @pytest.fixture