rustdoc --test does not work on windows

[alexcrichton]

$ echo 'pub fn foo() {}' > foo.rs
$ rustdoc --test foo.rs
foo.rs:1:1 1:1 error can't find crate for `std`
foo.rs:1 pub fn foo() {}
         ^
error: aborting due to previous error

[alexcrichton]

Dupe of #13259