Test UI worker interaction

  $ cat > t.py <<EOF

  > from __future__ import absolute_import, print_function

  > from mercurial import (

  >     cmdutil,

  >     ui as uimod,

  >     worker,

  > )

  > def runme(ui, args):

  >     for arg in args:

  >         ui.status('run\n')

  >         yield 1, arg

  > cmdtable = {}

  > command = cmdutil.command(cmdtable)

  > @command('test', [], 'hg test [COST]')

  > def t(ui, repo, cost=1.0):

  >     cost = float(cost)

  >     ui.status('start\n')

  >     runs = worker.worker(ui, cost, runme, (ui,), range(8))

  >     for n, i in runs:

  >         pass

  >     ui.status('done\n')

  > EOF

  $ abspath=`pwd`/t.py

  $ hg init

Run tests with worker enable by forcing a heigh cost

  $ hg --config "extensions.t=$abspath" test 100000.0

  start

  run

  run

  run

  run

  run

  run

  run

  run

  done

Run tests without worker by forcing a low cost

  $ hg --config "extensions.t=$abspath" test 0.0000001

  start

  run

  run

  run

  run

  run

  run

  run

  run

  done