[libcxx] [test] D27024: Fix MSVC warning C4389 "signed/unsigned mismatch", part 11/12.

Change "unsigned n = 0;" to "int n = 0;". It's being compared to int elements and ptrdiff_t distances.

test/std/containers/sequences/forwardlist/forwardlist.cons/move.pass.cpp
This one's a little special, but not really. "*i == n" is comparing MoveOnly to n.
MoveOnly is implicitly constructible from int, so int is the correct type to use here.

llvm-svn: 288748