patch 8.1.1619: tests are not run with GUI on Travis

Problem:    Tests are not run with GUI on Travis.
Solution:   Add a testgui job. (Ozaki Kiichi, closes #4609)
diff --git a/src/testdir/test_highlight.vim b/src/testdir/test_highlight.vim
index f21ebd7..2499543 100644
--- a/src/testdir/test_highlight.vim
+++ b/src/testdir/test_highlight.vim
@@ -601,10 +601,18 @@
 " This test must come before the Test_cursorline test, as it appears this
 " defines the Normal highlighting group anyway.
 func Test_1_highlight_Normalgroup_exists()
-  " MS-Windows GUI sets the font
-  if !has('win32') || !has('gui_running')
-    let hlNormal = HighlightArgs('Normal')
+  let hlNormal = HighlightArgs('Normal')
+  if !has('gui_running')
     call assert_match('hi Normal\s*clear', hlNormal)
+  elseif has('gui_gtk2') || has('gui_gnome') || has('gui_gtk3')
+    " expect is DEFAULT_FONT of gui_gtk_x11.c
+    call assert_match('hi Normal\s*font=Monospace 10', hlNormal)
+  elseif has('gui_motif') || has('gui_athena')
+    " expect is DEFAULT_FONT of gui_x11.c
+    call assert_match('hi Normal\s*font=7x13', hlNormal)
+  elseif has('win32')
+    " expect any font
+    call assert_match('hi Normal\s*font=.*', hlNormal)
   endif
 endfunc