patch 8.0.0277: the GUI test may trigger fontconfig and take a long time

Problem:    The GUI test may trigger fontconfig and take a long time.
Solution:   Set $XDG_CACHE_HOME. (Kazunobu Kuriyama)

src/testdir/test_gui.vim

@@ -10,7 +10,11 @@ func SetUp()
     set guifont=Courier\ 10\ Pitch/8/-1/5/50/0/0/0/0/0
   endif
 
-  " Gnome insists on creating $HOME/.gnome2/..
+  " 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 == ''
+    let $XDG_CACHE_HOME = g:tester_HOME . '/.cache'
+  endif
   call mkdir('Xhome')
   let $HOME = fnamemodify('Xhome', ':p')
 endfunc

src/testdir/unix.vim

@@ -2,4 +2,9 @@
 " Always use "sh", don't use the value of "$SHELL".
 set shell=sh
 
+" While some tests overwrite $HOME to prevent them from polluting user files,
+" we need to remember the original value so that we can tell external systems
+" where to ask about their own user settings.
+let g:tester_HOME = $HOME
+
 source setup.vim

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    277,
 /**/
     276,
 /**/