Remove TODO as it is covered with current test suite

Author: Ivanmatthew

crates/memofs/src/lib.rs

@@ -657,6 +657,4 @@ mod test {
         let err = vfs.canonicalize(&file_path).unwrap_err();
         assert_eq!(err.kind(), io::ErrorKind::NotFound);
     }
-
-    // TODO: Add a test to check whether new_default uses StdBackend.
 }