Problem: System test failing when using X11 forwarding.
Solution: Set $XAUTHORITY before changing $HOME. (closes#1812)
Also use a better check for the exit value.
Problem: It is not possible to use plugins in an "after" directory to tune
the behavior of a package.
Solution: First load plugins from non-after directories, then packages and
finally plugins in after directories.
Reset 'loadplugins' before executing --cmd arguments.
Problem: The list_lbr_utf8 test fails if ~/.vim/syntax/c.vim sets
'textwidth' to a non-zero value. (Oyvind A. Holm)
Solution: Add a setup.vim file that sets 'runtimepath' and $HOME to a safe
value. (partly by Christian Brabandt, closes#912)
