tests/test-revert-unknown.t

tests: allow optional output when `test-lfs-server` is sent SIGTERM

I guess this has only ever been tested on Windows, but on macOS, the test was
failing with changes like:

    +++ /private/tmp/mercurial-ci/tests/test-lfs-test-server.t#git-server.err
    @@ -858,6 +858,7 @@
     (Restart the server in a different location so it no longer has the content)

       $ "$PYTHON" $RUNTESTDIR/killdaemons.py $DAEMON_PIDS
    +  $TESTTMP.sh: line 153: 38639 Terminated: 15          lfs-test-server > lfs-server.log 2>&1

     #if hg-server
       $ cat $TESTTMP/access.log $TESTTMP/errors.log

  $ hg init repo
  $ cd repo
  $ touch unknown

  $ touch a
  $ hg add a
  $ hg ci -m "1"

  $ touch b
  $ hg add b
  $ hg ci -m "2"

Should show unknown

  $ hg status
  ? unknown
  $ hg revert -r 0 --all
  removing b

Should show unknown and b removed

  $ hg status
  R b
  ? unknown

Should show a and unknown

  $ ls -A
  .hg
  a
  unknown