Fix src/test/run-pass/variadic-ffi.rs test

[nagisa]

This test is ignored on MSVC because MSVC does not have a sprintf function.

We have a variadic function defined in test helpers, which should be used in this test as well. See mir_trans_calls_variadic for an example.

[luthfianto]

The #[link(name = "rust_test_helpers")]? What to do with it?

[nagisa]

@rilut It should be copied over along with the extern { fn rust_interesting_average… } block.