tests/test-mq-qsave.t

run-tests: detect HGWITHRUSTEXT value

Without this, some manual check in tests/test-dirstate.t could get confused by
the lack of `rust` in module policy and break the test.

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

  $ hg init repo
  $ cd repo

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

  $ hg qnew -mmqbase mqbase

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