patch 8.0.0805: GUI test fails with gnome2

Problem:    GUI test fails with gnome2.
Solution:   Set $HOME to an existing directory.
diff --git a/src/testdir/setup.vim b/src/testdir/setup.vim
index 4b6c089..6299852 100644
--- a/src/testdir/setup.vim
+++ b/src/testdir/setup.vim
@@ -14,5 +14,7 @@
   endif
 
   " Make sure $HOME does not get read or written.
-  let $HOME = '/does/not/exist'
+  " It must exist, gnome tries to create $HOME/.gnome2
+  let $HOME = getcwd() . '/XfakeHOME'
+  call mkdir($HOME)
 endif