tests/test-nointerrupt.t

run-tests: fix `HGTESTEXTRAEXTENSIONS` with py3

Since `extensions` was a str and `section` bytes, it never populated anything.
If it had, it would have put bytes into the environment dictionary that is all
str.  As everything starts and ends as str, remove the incomplete attempt at
byteification.  It doesn't appear that we had any test coverage of this bit of
code, so also add a non-extension config to make sure it is filtered out
properly.

Differential Revision: https://phab.mercurial-scm.org/D9557

#require no-windows

Dummy extension simulating unsafe long running command
  $ cat > sleepext.py <<EOF
  > import itertools
  > import time
  > 
  > from mercurial.i18n import _
  > from mercurial import registrar
  > 
  > cmdtable = {}
  > command = registrar.command(cmdtable)
  > 
  > @command(b'sleep', [], _(b'TIME'), norepo=True)
  > def sleep(ui, sleeptime=b"1", **opts):
  >     with ui.uninterruptible():
  >         for _i in itertools.repeat(None, int(sleeptime)):
  >             time.sleep(1)
  >         ui.warn(b"end of unsafe operation\n")
  >     ui.warn(b"%s second(s) passed\n" % sleeptime)
  > EOF

Kludge to emulate timeout(1) which is not generally available.
  $ cat > timeout.py <<EOF
  > from __future__ import print_function
  > import argparse
  > import signal
  > import subprocess
  > import sys
  > import time
  > 
  > ap = argparse.ArgumentParser()
  > ap.add_argument('-s', nargs=1, default='SIGTERM')
  > ap.add_argument('duration', nargs=1, type=int)
  > ap.add_argument('argv', nargs='*')
  > opts = ap.parse_args()
  > try:
  >     sig = int(opts.s[0])
  > except ValueError:
  >     sname = opts.s[0]
  >     if not sname.startswith('SIG'):
  >         sname = 'SIG' + sname
  >     sig = getattr(signal, sname)
  > proc = subprocess.Popen(opts.argv)
  > time.sleep(opts.duration[0])
  > proc.poll()
  > if proc.returncode is None:
  >     proc.send_signal(sig)
  >     proc.wait()
  >     sys.exit(124)
  > EOF

Set up repository
  $ hg init repo
  $ cd repo
  $ cat >> $HGRCPATH << EOF
  > [extensions]
  > sleepext = ../sleepext.py
  > EOF

Test ctrl-c
  $ python $TESTTMP/timeout.py -s INT 1 hg sleep 2
  interrupted!
  [124]

  $ cat >> $HGRCPATH << EOF
  > [experimental]
  > nointerrupt = yes
  > EOF

  $ python $TESTTMP/timeout.py -s INT 1 hg sleep 2
  interrupted!
  [124]

  $ cat >> $HGRCPATH << EOF
  > [experimental]
  > nointerrupt-interactiveonly = False
  > EOF

  $ python $TESTTMP/timeout.py -s INT 1 hg sleep 2
  shutting down cleanly
  press ^C again to terminate immediately (dangerous)
  end of unsafe operation
  interrupted!
  [124]