Fixed lfs_file_truncate issue where internal state may not be flushed

This was caused by the new lfs_file_rawseek optimization that can skip
flushing when calculated file->pos is unchanged combined with an
implicit expectation in lfs_file_truncate that lfs_file_rawseek
unconditionally sets file->pos.

Because of this assumption, lfs_file_truncate could leave file->pos in
an outdated state while changing the internal file metadata. Humorously,
this was always gauranteed to trigger the skip in lfs_file_rawseek when
we try to restore the file->pos, leaving the file->cache used to do the
CTZ skip-list lookup in a potentially bad state.

The easiest fix is to just update file->pos correctly. Note we don't
want to explicitly flush since we can leverage the same noop
optimization if we truncate to the file position. Which I've added a
test for.

lfs.c

@@ -3106,6 +3106,9 @@ static int lfs_file_rawtruncate(lfs_t *lfs, lfs_file_t *file, lfs_off_t size) {
             return err;
         }
 
+        // need to set pos/block/off consistently so seeking back to
+        // the old position does not get confused
+        file->pos = size;
         file->ctz.head = file->block;
         file->ctz.size = size;
         file->flags |= LFS_F_DIRTY | LFS_F_READING;