Debug tests

@mortenvp @jpihl Sak is all green now after some trial-and-error debugging on iOS. I switched the order of some lines in test_buffer.cpp, because the created std::vectors had sizes like 4294967056. Totally insane ;)
I also had to disable the unit test for file_input_stream, because that will not work on iOS and OpenWrt. That class is not used in other projects, so I think it is better to sacrifice it in exchange for running all the other tests of the core functionality (which should be tested on all platforms!).
What do you think?