QUnit.newMwEnvironment: Disable mw.log#warn while copying mw.config

QUnit.newMwEnvironment: Disable mw.log#warn while copying mw.config

As of 89a8fe4, mw.log#warn is available in non-debug mode so that
e.g. MWDeprecationWarning are emitted whenever such property is
accessed.

Due to the way QUnit.newMwEnvironment makes a copy of the live
config for the duration of a test run (using $.extend) this
causes all the accessors to be triggered.

As a result the qunit log is inflated to 100s of MWDeprecationWarning
entries before every single test:

 http://integration.wikimedia.org/ci/job/mediawiki-core-qunit/16525/console

Using Object.create (with polyfill) doesn't work since mw.Map#get
only resolves own properties.

Change-Id: I7285c56bd1ae7ef2efae15ee0427eeb77bc240ac