test scripts don't support unshelve

While investigating bug #850594 I tried writing the corresponding test-script:

$ bzr init .
$ mkdir dir/
$ echo "this line represents editing" >> dir/file
$ bzr add
$ bzr shelve -m "work on file" dir/file
# Shelve adding file "dir/file"? [yNfq?] y
# Shelve adding file "dir"? [yNfq?] n
# Selected changes:
# -D dir/file
# Shelve 1 change(s)? [yNfq?] y
<yny
$ bzr ci -m "setup dir"
$ bzr unshelve

Trying it with './bzr test-script --null-output bug.script' fails with:

FAIL: bzrlib.cmd_test_script.Test.test_it
    log: {{{
0.131 creating repository in file:///tmp/testbzr-sds4kh.tmp/.bzr/.
0.134 creating branch <bzrlib.branch.BzrBranchFormat7 object at 0x2233990> in file:///tmp/testbzr-sds4kh.tmp/
0.141 trying to create missing lock '/tmp/testbzr-sds4kh.tmp/.bzr/checkout/dirstate'
0.142 opening working tree '/tmp/testbzr-sds4kh.tmp'
0.149 run bzr: ['init', '.']
0.149 bazaar version: 2.5.0dev2
0.149 bzr arguments: [u'init', u'.']
0.151 encoding stdout as sys.stdout encoding 'UTF-8'
0.154 creating repository in file:///tmp/testbzr-sds4kh.tmp/bzrlib.cmd_test_script.Test.test_it/work/.bzr/.
0.157 creating branch <bzrlib.branch.BzrBranchFormat7 object at 0x2c9a7d0> in file:///tmp/testbzr-sds4kh.tmp/bzrlib.cmd_test_script.Test.test_it/work/
0.161 trying to create missing lock '/tmp/testbzr-sds4kh.tmp/bzrlib.cmd_test_script.Test.test_it/work/.bzr/checkout/dirstate'
0.161 opening working tree '/tmp/testbzr-sds4kh.tmp/bzrlib.cmd_test_script.Test.test_it/work'
0.166 opening working tree '/tmp/testbzr-sds4kh.tmp/bzrlib.cmd_test_script.Test.test_it/work'
0.181 output:
'Created a standalone tree (format: 2a)\n'
0.181 run bzr: ['add']
0.181 bazaar version: 2.5.0dev2
0.181 bzr arguments: [u'add']
0.183 encoding stdout as sys.stdout encoding 'UTF-8'
0.185 opening working tree '/tmp/testbzr-sds4kh.tmp/bzrlib.cmd_test_script.Test.test_it/work'
0.189 skip control directory '.bzr'
0.190 creating config directory: '/tmp/testbzr-sds4kh.tmp/bzrlib.cmd_test_script.Test.test_it/home/.bazaar'
0.194 output:
'adding dir\nadding dir/file\n'
0.195 run bzr: ['shelve', '-m', 'work on file', 'dir/file']
0.195 bazaar version: 2.5.0dev2
0.195 bzr arguments: [u'shelve', u'-m', u'work on file', u'dir/file']
0.196 encoding stdout as sys.stdout encoding 'UTF-8'
0.199 opening working tree '/tmp/testbzr-sds4kh.tmp/bzrlib.cmd_test_script.Test.test_it/work'
0.204 Traceback (most recent call last):
  File "/home/vila/src/bzr/trunk/bzrlib/commands.py", line 1230, in run_bzr_catch_user_errors
    return run_bzr(argv)
  File "/home/vila/src/bzr/trunk/bzrlib/commands.py", line 1123, in run_bzr
    ret = run(*run_argv)
  File "/home/vila/src/bzr/trunk/bzrlib/commands.py", line 677, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/home/vila/src/bzr/trunk/bzrlib/commands.py", line 699, in run
    return self._operation.run_simple(*args, **kwargs)
  File "/home/vila/src/bzr/trunk/bzrlib/cleanup.py", line 135, in run_simple
    self.cleanups, self.func, *args, **kwargs)
  File "/home/vila/src/bzr/trunk/bzrlib/cleanup.py", line 165, in _do_with_cleanups
    result = func(*args, **kwargs)
  File "/home/vila/src/bzr/trunk/bzrlib/builtins.py", line 6254, in run
    shelver.run()
  File "/home/vila/src/bzr/trunk/bzrlib/shelf_ui.py", line 205, in run
    if self.prompt_bool(self.reporter.prompt_change(change)):
  File "/home/vila/src/bzr/trunk/bzrlib/shelf_ui.py", line 291, in prompt_bool
    char = self.prompt(question + prompt)
  File "/home/vila/src/bzr/trunk/bzrlib/shelf_ui.py", line 265, in prompt
    raise errors.BzrError(gettext("You need a controlling terminal."))
BzrError: You need a controlling terminal.

0.204 errors:
'bzr: ERROR: You need a controlling terminal.\n'
}}}