patch 8.0.0447: getting font name does not work on X11

Problem:    Getting font name does not work on X11.
Solution:   Implement gui_mch_get_fontname() for X11.  Add more GUI tests.
            (Kazunobu Kuriyama)
diff --git a/src/testdir/gui_init.vim b/src/testdir/gui_init.vim
index bc5e33c..42b2bca 100644
--- a/src/testdir/gui_init.vim
+++ b/src/testdir/gui_init.vim
@@ -2,4 +2,5 @@
 
 if has('gui_athena') || has('gui_motif') || has('gui_gtk2') || has('gui_gtk3')
   set guiheadroom=0
+  set guioptions+=p
 endif