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_timers.vim b/src/testdir/test_timers.vim
index de13bcb..ee86e2a 100644
--- a/src/testdir/test_timers.vim
+++ b/src/testdir/test_timers.vim
@@ -334,7 +334,7 @@
 endfunc
 
 func Test_error_in_timer_callback()
-  if !has('terminal') || (has('win32') && has('gui_running'))
+  if !has('terminal') || has('gui_running')
     throw 'Skipped: cannot run Vim in a terminal window'
   endif