patch 8.0.0278: GUI test fails on MS-Windows

Problem:    GUI test fails on MS-Windows.
Solution:   Check that tester_HOME exists.

src/testdir/test_gui.vim

@@ -10,9 +10,10 @@ func SetUp()
     set guifont=Courier\ 10\ Pitch/8/-1/5/50/0/0/0/0/0
   endif
 
-  " Gnome insists on creating $HOME/.gnome2/, and we're going to abide by
-  " that, taking care to have the font cache work normally.
-  if $XDG_CACHE_HOME == ''
+  " Gnome insists on creating $HOME/.gnome2/, set $HOME to avoid changing the
+  " actual home directory.  But avoid triggering fontconfig by setting the
+  " cache directory.  Only needed for Unix.
+  if $XDG_CACHE_HOME == '' && exists('g:tester_HOME')
     let $XDG_CACHE_HOME = g:tester_HOME . '/.cache'
   endif
   call mkdir('Xhome')

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    278,
 /**/
     277,
 /**/