test(ngMock): workaround issue with negative timestamps

[brettporter]

In some specific timezones and operating systems, it seems that
getTimezoneOffset() can return an incorrect value for negative timestamps, as
described in #5017. While this isn't something easily fixed in the mock code,
the tests can avoid that particular timeframe by using a positive timestamp.

Closes #5017

[mary-poppins]

Thanks for the PR! Please check the items below to help us merge this faster. See the contributing docs for more information.

• Uses the issue template (#6730)

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[mary-poppins]

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[brettporter]

I'm sure I signed the CLA with this address last time, but done again...

[mary-poppins]

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[IgorMinar]

thanks! I have no good way of testing this so let us know if there are any further problems.