NB: the `!(flags & SOPT_GLOBAL)` exception is for 'statusline'.
Because `:set statusline=...` sets the global value for _all_ windows,
`:setlocal` is the best we can do there. This is a one-of-a-kind option
that doesn't work like any other option.
Use unique filenames to avoid test conflicts.
Use read_file() instead of io.popen(), to ensures the file is closed.
Use helpers.rmdir(), it is far more robust than lfs.
closes#7911
