OPT(TST): cache ds005 collection and return a clone upon invocation

Creating a collection apparently is more expensive.
Clone() is needed since some tests do modification inplace. If they did not
it was possible to declare this fixture as (scope="module"). So we need to return
a clone each time.  This causes full run time of 9 sec vs 30 sec.

Disclaimer:there is still some nasty "inplace" changes happening probably or
clone() being not sufficient.  For me
  python -m pytest -s -v bids/analysis/tests/test_transformations.py::test_{filter,replace}
causes test_replace to fail.

bids/analysis/tests/test_transformations.py

@@ -18,17 +18,22 @@
 NRUNS = 3
 SCAN_LENGTH = 480
 
+cached_collections = {}
+
 
 @pytest.fixture
 def collection():
+    if 'ds005' in cached_collections:
+        return cached_collections['ds005'].clone()
     layout_path = join(get_test_data_path(), 'ds005')
     layout = BIDSLayout(layout_path)
-    collection = layout.get_collections('run', types=['events'],
-                                        scan_length=SCAN_LENGTH,
-                                        merge=True,
-                                        sampling_rate=10,
-                                        subject=SUBJECTS
-                                        )
+    cached_collections['ds005'] = collection \
+        = layout.get_collections('run', types=['events'],
+                                 scan_length=SCAN_LENGTH,
+                                 merge=True,
+                                 sampling_rate=10,
+                                 subject=SUBJECTS
+                                 )
     return collection