Use std::path::MAIN_SEPARATOR

tests/decode_libtiffpic.rs

@@ -115,7 +115,7 @@ fn main() {
         let Some(path) = entry.path().to_str() else {
             continue;
         };
-        let path = path.replace('\\', "/"); // always use UNIX path separators
+        let path = path.replace(std::path::MAIN_SEPARATOR, "/"); // always use UNIX path separators
 
         // Mark this path as consumed in either case by removing it.
         let Some(should_decode) = candidates.remove(path.as_str()) else {