Disable fs-mistrust's simple_cases() unit test on non-Unix.

This patch disables the simple_cases() test on non-Unix platforms and
hides the LinkType type import on non-Unix where we won't be testing
symbolic link features.

See: tpo/core/arti#557.

crates/fs-mistrust/src/lib.rs

@@ -815,8 +815,12 @@ impl<'a> Verifier<'a> {
 mod test {
     #![allow(clippy::unwrap_used)]
     use super::*;
-    use testing::{Dir, LinkType};
+    use testing::Dir;
 
+    #[cfg(target_family = "unix")]
+    use testing::LinkType;
+
+    #[cfg(target_family = "unix")]
     #[test]
     fn simple_cases() {
         let d = Dir::new();