tests/test-mq-qsave.t

obsolete: introduce an `_enabled` switch to disable the feature by default

Obsolete markers wide-usage and propagation should be avoided by default until
the obsolete feature is more mature.

This changeset introduce the `_enable` variable and prevent the creation of
obsolete marker if the feature is set to `False` (the default).

More limitation comes in followup changesets.

  $ 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