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/pybind11_tests.h

@@ -0,0 +1,7 @@
+#include <pybind11/pybind11.h>
+#include <iostream>
+
+using std::cout;
+using std::endl;
+
+namespace py = pybind11;