Problem:    Running tests may pollute shell history. (Manuel Ortega)
Solution:   Make $HISTFILE empty.

Problem:    MS-Windows: tests still hang.
Solution:   Only create the XfakeHOME directory if it does not exist yet.

Problem:    Tests may try to create XfakeHOME twice.
Solution:   Avoid loading setup.vim twice.

Problem:    GUI test fails with gnome2.
Solution:   Set $HOME to an existing directory.

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:    Small build can't run tests.
Solution:   Don't try setting 'packpath'.

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)