gh-265: calculate fixture once per testing session (#266)

Follows up #241. See explanation
of what the `scope` parameter does for `pytest.fixture`. Seems silly to
not use it (although in this case the speed-up will be minimal).

tests/conftest.py

@@ -1,7 +1,7 @@
 import pytest
 
 
-@pytest.fixture
+@pytest.fixture(scope="session")
 def rng():
     import numpy as np