Add a conftest to prefer faster tests

This uses pytest_collection_modifyitems for pytest's own tests to order
them, preferring faster ones via quick'n'dirty heuristics only for now.

testing/test_modimport.py

@@ -6,6 +6,8 @@ import py
 import _pytest
 import pytest
 
+pytestmark = pytest.mark.slow
+
 MODSET = [
     x
     for x in py.path.local(_pytest.__file__).dirpath().visit("*.py")