Fixed accidental truncate after seek on inline files

The cause was mistakenly setting file->ctz.size directly instead of
file->pos, which file->ctz.size gets overwritten with later in
lfs_file_flush.

Also added better seek test cases specifically for inline files. This
should also catch most of the inline corner cases related to
lfs_file_size/lfs_file_tell.

Found by ebinans

lfs.c

@@ -2548,7 +2548,7 @@ relocate:
                 }
             }
         } else {
-            file->ctz.size = lfs_max(file->pos, file->ctz.size);
+            file->pos = lfs_max(file->pos, file->ctz.size);
         }
 
         // actual file updates