mw.log.deprecate: Use mw.log.warn instead of log.warn

mw.log.deprecate: Use mw.log.warn instead of log.warn

In qunit/testrunner.js, freshConfigCopy() temporarily clears
mw.log.warn to avoid a flood of spurious deprecation warnings.

However, as of 89a8fe4, the console was once again flooded with
deprecated notices when in debug mode because in debug mode we
load mediawiki.log.js which redefines the mw.log object and as
such the local reference here kept pointing to the original.

Change-Id: I2b560aac9c0b929c82bb628760c194c0476fe1a9