patch 8.1.0820: test for sending large data over channel sometimes fails

Problem:    Test for sending large data over channel sometimes fails.
Solution:   Handle that the job may have finished early.  Also fix that file
            changed test doesn't work in the GUI and reduce flakyness. (Ozaki
            Kiichi, closes #3861)
diff --git a/src/testdir/test_filechanged.vim b/src/testdir/test_filechanged.vim
index ab34420..43f752d 100644
--- a/src/testdir/test_filechanged.vim
+++ b/src/testdir/test_filechanged.vim
@@ -90,7 +90,7 @@
 endfunc
 
 func Test_file_changed_dialog()
-  if !has('unix')
+  if !has('unix') || has('gui_running')
     return
   endif
   au! FileChangedShell