terminal_get_line_attributes() had this bug for a long time, though it
likely had no effect visible to users.

ref #9028
ref 60f845ca55a1

By historical accident, Nvim defaults to background=light. So on a dark
background, `:colorscheme default` looks completely wrong.

The "smart" logic that Vim uses is confusing for anyone who uses Vim on
multiple platforms, so rather than mimic that, pick the (hopefully) most
common default.

- Since Neovim is dark-powered, we assume most users have dark backgrounds.
- Most of the GUIs tend to have a dark background by default.

ref #6289

- window_split_tab_spec.lua: Put cursor at bottom of :terminal buffer so
  that it follows output.
- inccommand_spec.lua: Increase timeout to allow 2nd retry.
- Timer tests are less reliable on Travis CI macOS 10.12/10.13.
  ref #6829
  ref e39dade80b02
  ref de13113dc16e
  ref https://github.com/neovim/neovim/pull/9095#issuecomment-429603452
  > We don't guarantee that a X ms timer is triggered during Y ms sleep
  > for any X<Y, though I would expect the load to be really bad for this
  > to happen with X=10ms, Y=40ms.

The default timeout of 10s only allows 1 retry.

- fix: Use "set PROMPT=$" trick to satisfy screen:expect{any='%$'}
- speedup: Use cmd.exe instead of powershell.