Fix tests with Rust 1.55

Per rust-lang/rust#85746, Rust 1.55 has stopped
returning io::ErrorKind::Other from within the standard library and
instead returns better, more-specific error codes.  This broke a test of
ours that matched on `Other`.

Address this by relaxing the test and only checking for an error, not
the specific error type.  Fixing this properly seems quite hard at this
point per the added TODO.

Author: jmmv

std/src/storage/fs.rs

@@ -251,7 +251,11 @@ mod tests {
         let file = dir.path().join("not-a-dir");
         write_file(&file, &[]);
         let drive = DirectoryDrive::new(&file).unwrap();
-        assert_eq!(io::ErrorKind::Other, block_on(drive.enumerate()).unwrap_err().kind());
+        // TODO(jmmv): Check for the specific error that's returned.  We used to check against
+        // `Other` but Rust 1.55 started returning `NotADirectory` instead -- and unfortunately
+        // using the latter relies on an unstable feature.  So addressing this is non-trivial
+        // right now, but will be over time.
+        block_on(drive.enumerate()).unwrap_err();
     }
 
     #[test]