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_callbacks.cpp

@@ -0,0 +1,103 @@
+/*
+    tests/test_callbacks.cpp -- callbacks
+
+    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"
+#include "constructor_stats.h"
+#include <pybind11/functional.h>
+
+
+bool test_callback1(py::object func) {
+    func();
+    return false;
+}
+
+int test_callback2(py::object func) {
+    py::object result = func("Hello", 'x', true, 5);
+    return result.cast<int>();
+}
+
+void test_callback3(const std::function<int(int)> &func) {
+    cout << "func(43) = " << func(43)<< std::endl;
+}
+
+std::function<int(int)> test_callback4() {
+    return [](int i) { return i+1; };
+}
+
+py::cpp_function test_callback5() {
+    return py::cpp_function([](int i) { return i+1; },
+       py::arg("number"));
+}
+
+int dummy_function(int i) { return i + 1; }
+int dummy_function2(int i, int j) { return i + j; }
+std::function<int(int)> roundtrip(std::function<int(int)> f) { 
+    if (!f)
+        std::cout << "roundtrip (got None).." << std::endl;
+    else
+        std::cout << "roundtrip.." << std::endl;
+    return f;
+}
+
+void test_dummy_function(const std::function<int(int)> &f) {
+    using fn_type = int (*)(int);
+    auto result = f.target<fn_type>();
+    if (!result) {
+        std::cout << "could not convert to a function pointer." << std::endl;
+        auto r = f(1);
+        std::cout << "eval(1) = " << r << std::endl;
+    } else if (*result == dummy_function) {
+        std::cout << "argument matches dummy_function" << std::endl;
+        auto r = (*result)(1);
+        std::cout << "eval(1) = " << r << std::endl;
+    } else {
+        std::cout << "argument does NOT match dummy_function. This should never happen!" << std::endl;
+    }
+}
+
+struct Payload {
+    Payload() {
+        print_default_created(this);
+    }
+    ~Payload() {
+        print_destroyed(this);
+    }
+    Payload(const Payload &) {
+        print_copy_created(this);
+    }
+    Payload(Payload &&) {
+        print_move_created(this);
+    }
+};
+
+void init_ex_callbacks(py::module &m) {
+    m.def("test_callback1", &test_callback1);
+    m.def("test_callback2", &test_callback2);
+    m.def("test_callback3", &test_callback3);
+    m.def("test_callback4", &test_callback4);
+    m.def("test_callback5", &test_callback5);
+
+    /* Test cleanup of lambda closure */
+
+    m.def("test_cleanup", []() -> std::function<void(void)> { 
+        Payload p;
+
+        return [p]() {
+            /* p should be cleaned up when the returned function is garbage collected */
+        };
+    });
+
+    /* Test if passing a function pointer from C++ -> Python -> C++ yields the original pointer */
+    m.def("dummy_function", &dummy_function);
+    m.def("dummy_function2", &dummy_function2);
+    m.def("roundtrip", &roundtrip);
+    m.def("test_dummy_function", &test_dummy_function);
+    // Export the payload constructor statistics for testing purposes:
+    m.def("payload_cstats", &ConstructorStats::get<Payload>);
+}