Improve test setup: add Makefile for "make test"

This auto-installs dependencies.

Additional fixes:
 - Set $HOME before calling vim, otherwise a user's plugins would be
   loaded, too.
 - Install textobj-user (used in vimrc, but not on travis (yet?!)).
 - Set `viminfo=`.

Fixes https://github.com/idbrii/vim-endoscope/issues/1