consolidate to one pytest config (#2020)

don't even try collecting properties/ as this raises if hypothesis not
installed

Author: max-sixty

pytest.ini

@@ -3,6 +3,7 @@ universal = 1
 
 [tool:pytest]
 python_files=test_*.py
+testpaths=xarray/tests
 
 [flake8]
 max-line-length=79