Replace std::cout with py::print in tests

With this change both C++ and Python write to sys.stdout which resolves
the capture issues noted in #351. Therefore, the related workarounds are
removed.

tests/pybind11_tests.h

@@ -1,12 +1,8 @@
 #pragma once
 #include <pybind11/pybind11.h>
-#include <iostream>
 #include <functional>
 #include <list>
 
-using std::cout;
-using std::endl;
-
 namespace py = pybind11;
 using namespace pybind11::literals;