Update default paths in test_runtime_paths

neovim/neovim#1005 changes the default runtime paths for Neovim, this commit updates them in the runtime test, and adds a an alternative default (for the case where the runtime dir exists).

- Neovim runtime path is now PREFIX/share/nvim
  instead of PREFIX/share/vim
- Neovim rtp may differ if the runtime folder is
  installed under the default path - updated the
  test to allow both options