tests/test-gendoc-it.t

testlib: add a small scrip to help process to synchronise using file

Creating and waiting for files is a robust way to synchronise two processes
running concurrently. We already use this approach in various tests. I am adding
a official script to do so before adding more usage of this.

Differential Revision: https://phab.mercurial-scm.org/D8189

#require docutils gettext

  $ $TESTDIR/check-gendoc it
  checking for parse errors