patch 8.0.1339: no test for what 8.0.1335 fixes

Problem:    No test for what 8.0.1335 fixes.
Solution:   Add a test. (Yasuhiro Matsumoto, closes #2373)

src/testdir/test_writefile.vim

@@ -100,3 +100,11 @@ func Test_writefile_sync_arg()
   call writefile(['two'], 'Xtest', 'S')
   call delete('Xtest')
 endfunc
+
+func Test_writefile_sync_dev_stdout()
+  if !has('unix')
+    return
+  endif
+  " Just check that this doesn't cause an error.
+  call writefile(['one'], '/dev/stdout')
+endfunc

src/version.c

@@ -771,6 +771,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1339,
 /**/
     1338,
 /**/