patch 8.0.0806: tests may try to create XfakeHOME twice

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

Author: brammool

src/testdir/setup.vim

@@ -1,5 +1,13 @@
 " Common preparations for running tests.
 
+" Only load this once.
+if 1
+  if exists('s:did_load')
+    finish
+  endif
+  let s:did_load = 1
+endif
+
 " Make sure 'runtimepath' and 'packpath' does not include $HOME.
 set rtp=$VIM/vimfiles,$VIMRUNTIME,$VIM/vimfiles/after
 if has('packages')

src/version.c

@@ -769,6 +769,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    806,
 /**/
     805,
 /**/