c++0x tests fixes

[Paolo Carlini]

Hi,

> So I would simply propose to hide the explicit instantiation with a #ifndef __GLIBCXX_EXPERIMENTAL_CXX0X__.
> 
> Ok ?

Sure. I would suggest also adding a one line comment about the issue, checking that is consistent with a related comment in an instantiation test for std::map (which I added when I noticed that in c++03 std::map is the only container which cannot be explicitly instantiated for non-DefaultConstructible mapped type because of operator[])

Paolo