[gulpfile.js] Move the time-zone hack to the `testing-pre` task, such that *all* tests work regardless of the current time-zone
[gulpfile.js] Move the time-zone hack to the `testing-pre` task, such that *all* tests work regardless of the current time-zone

Currently only the `gulp unittest` task actually set the time-zone, which means that locally I'm now getting failures with e.g. `gulp test`.

*Please note:* I firmly believe that the unit-tests in question should be re-written, since even with this patch applied there's failures when running http://localhost:8888/test/unit/unit_test.html directly in a browser.

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