Unit Tests: Add method to randomize dates

[iamgabrielma]

At the moment we're hardcoding dates in all our tests, for example ProductVariationMapperTests or ProductVariationsRemoteTests, for multiple parameters like dateCreated, dateModified, dateOnSaleStart, dateOnSaleEnd

An idea from https://github.com/woocommerce/woocommerce-ios/pull/7080#pullrequestreview-1005173104 could be to randomize these, for example in our DateFormatter extension:

I think having a method for randomizing dates is actually a good thought! We could probably add a parameter in that method to make it possible to randomize a date from the past or the future, that will come in handy as well.

[peril-woocommerce[bot]]

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