Problem: Two tests in test_filechanged.vim are slow.
Solution: Sleep for shorter if the +nanotime feature is available.
(zeertzjq)
closes: vim/vim#1441883cd2c7bf0
Problem: Dialog for file changed outside of Vim not tested.
Solution: Add a test. Move FileChangedShell test. Add 'L' flag to
feedkeys().
5e66b42aae
Co-authored-by: Bram Moolenaar <Bram@vim.org>