Do backtrace string matching test only on some platforms.

mod test {

    use super::*;

    // We test this on "important" and "reliable" platforms only.

    //

    // This test case mainly is to ensure that we are using the backtrace module correctly, etc.,

    // which can be checked by doing it on one platform.

    //

    // Doing the test on on *all* platforms would simply expose us to the vagaries of platform

    // backtrace support.  Arti ought not to fail its tests just because someone is using a

    // platform with poor backtrace support.

    //

    // On the other hand, we *do* want to know that things are correct on platforms where we think

    // Rust backtraces work properly.

    //

    // So this list is a compromise.  See

    //   https://gitlab.torproject.org/tpo/core/arti/-/merge_requests/509#note_2803085

    #[cfg(any(

        target_os = "linux",

        target_os = "macos",

        target_os = "windows",

    ))]

    #[test]

    fn internal_macro_test() {

        let start_of_func = line!();