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
  " Just check that this doesn't cause an error.
  call writefile(['one'], '/dev/stdout')
  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,
/**/