Add Catch framework for testing embedding support and C++-side features

At this point, there is only a single test for interpreter basics.

Apart from embedding itself, having a C++ test framework will also
benefit the C++-side features by allowing them to be tested directly.

tests/test_embed/test_interpreter.cpp

@@ -0,0 +1,64 @@
+#include <pybind11/pybind11.h>
+#include <pybind11/eval.h>
+
+#include <catch.hpp>
+
+namespace py = pybind11;
+using namespace py::literals;
+
+class Widget {
+public:
+    Widget(std::string message) : message(message) { }
+    virtual ~Widget() = default;
+
+    std::string the_message() const { return message; }
+    virtual int the_answer() const = 0;
+
+private:
+    std::string message;
+};
+
+class PyWidget final : public Widget {
+    using Widget::Widget;
+
+    int the_answer() const override { PYBIND11_OVERLOAD_PURE(int, Widget, the_answer); }
+};
+
+PyObject *make_embedded_module() {
+    py::module m("widget_module");
+
+    py::class_<Widget, PyWidget>(m, "Widget")
+        .def(py::init<std::string>())
+        .def_property_readonly("the_message", &Widget::the_message);
+
+    return m.ptr();
+}
+
+py::object import_file(const std::string &module, const std::string &path, py::object globals) {
+    auto locals = py::dict("module_name"_a=module, "path"_a=path);
+    py::eval<py::eval_statements>(
+        "import imp\n"
+        "with open(path) as file:\n"
+        "    new_module = imp.load_module(module_name, file, path, ('py', 'U', imp.PY_SOURCE))",
+        globals, locals
+    );
+    return locals["new_module"];
+}
+
+TEST_CASE("Pass classes and data between modules defined in C++ and Python") {
+    PyImport_AppendInittab("widget_module", &make_embedded_module);
+    Py_Initialize();
+    {
+        auto globals = py::module::import("__main__").attr("__dict__");
+        auto module = import_file("widget", "test_interpreter.py", globals);
+        REQUIRE(py::hasattr(module, "DerivedWidget"));
+
+        auto py_widget = module.attr("DerivedWidget")("Hello, World!");
+        auto message = py_widget.attr("the_message");
+        REQUIRE(message.cast<std::string>() == "Hello, World!");
+
+        const auto &cpp_widget = py_widget.cast<const Widget &>();
+        REQUIRE(cpp_widget.the_answer() == 42);
+    }
+    Py_Finalize();
+}