tests/std.t: test #28: Change return value from 9 to 2

This is some old weirdness described in commit c6a3a03, also known as
99f62ca8-5676-11e0-aa82-b7a467e4e366 ("std.t: Set return value in a test
to 9", 2011-03-25 01:23:36 +0100). As I wrote in that log message:

  std.t: Set return value in a test to 9

  The test "Create bash script, file already exists, don’t use --force"
  originally contained the exit value 1, but die() seems to return some
  odd value. 9 = EBADF (Bad file number), and that makes no sense. I’d
  expect it would return 17 (EEXIST, File exists), but apparently not.

  Oh well, let’s see if it’s consistent on various systems.

It's changed now, I'll try to set it to 2 and see if it's the same
everywhere.

9a5163a0-0ce6-11e4-890b-c80aa9e67bbd