testsuite: test for fixed issue. Closes #7580.

Fixed by the privacy changes that allowed the `mod std {}` at the top
level of `std` to be non-`pub`.
auto merge of #9973 : huonw/rust/7580, r=luqmana

Fixed by the privacy changes that allowed the `mod std {}` at the top
level of `std` to be non-`pub`.