tests/test-gendoc-it.t

keepalive: ensure `close_all()` actually closes all cached connections

While debugging why LFS blob downloads are getting corrupted with workers, I
noticed that prior to spinning up the workers, the ConnectionManager has 2
connections to the server and calling `KeepAliveHandler.close_all()` left one
behind.  The reason is the value component of `self._cm.get_all().items()` is a
list, and `self._cm.remove()` modifies said list while the caller is iterating
over it.  Now `get_all()` is a deep copy of both the dict and lists in all
cases.

#require docutils gettext

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