Port tests to pytest

Use simple asserts and pytest's powerful introspection to make testing
simpler. This merges the old .py/.ref file pairs into simple .py files
where the expected values are right next to the code being tested.

This commit does not touch the C++ part of the code and replicates the
Python tests exactly like the old .ref-file-based approach.

tests/test_eval_call.py

@@ -0,0 +1,4 @@
+# This file is called from 'test_eval.py'
+
+if 'call_test2' in globals():
+    call_test2(y)