tests/test-mq-qsave.t

tests: replace mockmakedate function in test-amend.t

This is a follow up patch for D5554. This replaces mockmakedate function
by `tests/mockmakedate.py`.

Differential Revision: https://phab.mercurial-scm.org/D5577

  $ echo "[extensions]" >> $HGRCPATH
  $ echo "mq=" >> $HGRCPATH

  $ hg init

  $ echo 'base' > base
  $ hg ci -Ambase
  adding base

  $ hg qnew -mmqbase mqbase

  $ hg qsave
  $ hg qrestore 2
  restoring status: hg patches saved state