[libc++][test] std::variant test cleanup

* Add the conventional `return 0` to `main` in `variant.assign/conv.pass.cpp` and `variant.ctor/conv.pass.cpp`

* Fix some MSVC signed-to-unsigned conversion warnings by replacing `int` literarls with `unsigned int` literals

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@374723 91177308-0d34-0410-b5e6-96231b3b80d8