patch 8.0.1744: on some systems /dev/stdout isn't writable

Problem:    On some systems /dev/stdout isn't writable.
Solution:   Skip test if writing is not possible. (James McCoy, closes #2830)

src/testdir/test_writefile.vim

@@ -105,6 +105,10 @@ func Test_writefile_sync_dev_stdout()
   if !has('unix')
     return
   endif
+  if filewritable('/dev/stdout')
     " Just check that this doesn't cause an error.
     call writefile(['one'], '/dev/stdout')
+  else
+    throw 'Skipped: /dev/stdout is not writable'
+  endif
 endfunc

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1744,
 /**/
     1743,
 /**/