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_pickling.cpp

@@ -0,0 +1,51 @@
+/*
+    tests/test_pickling.cpp -- pickle support
+
+    Copyright (c) 2016 Wenzel Jakob <wenzel.jakob@epfl.ch>
+
+    All rights reserved. Use of this source code is governed by a
+    BSD-style license that can be found in the LICENSE file.
+*/
+
+#include "pybind11_tests.h"
+
+class Pickleable {
+public:
+    Pickleable(const std::string &value) : m_value(value) { }
+    const std::string &value() const { return m_value; }
+
+    void setExtra1(int extra1) { m_extra1 = extra1; }
+    void setExtra2(int extra2) { m_extra2 = extra2; }
+    int extra1() const { return m_extra1; }
+    int extra2() const { return m_extra2; }
+private:
+    std::string m_value;
+    int m_extra1 = 0;
+    int m_extra2 = 0;
+};
+
+void init_ex_pickling(py::module &m) {
+    py::class_<Pickleable>(m, "Pickleable")
+        .def(py::init<std::string>())
+        .def("value", &Pickleable::value)
+        .def("extra1", &Pickleable::extra1)
+        .def("extra2", &Pickleable::extra2)
+        .def("setExtra1", &Pickleable::setExtra1)
+        .def("setExtra2", &Pickleable::setExtra2)
+        // For details on the methods below, refer to
+        // http://docs.python.org/3/library/pickle.html#pickling-class-instances
+        .def("__getstate__", [](const Pickleable &p) {
+            /* Return a tuple that fully encodes the state of the object */
+            return py::make_tuple(p.value(), p.extra1(), p.extra2());
+        })
+        .def("__setstate__", [](Pickleable &p, py::tuple t) {
+            if (t.size() != 3)
+                throw std::runtime_error("Invalid state!");
+            /* Invoke the constructor (need to use in-place version) */
+            new (&p) Pickleable(t[0].cast<std::string>());
+
+            /* Assign any additional state */
+            p.setExtra1(t[1].cast<int>());
+            p.setExtra2(t[2].cast<int>());
+        });
+}