In order to use still use mocks with the generation of random strings in player requests, we need to use YoutubeParsingHelper.setSeedForVideoTests() method in every stream test.
This method is needed for YouTube stream tests, because when all YouTube tests are ran, the signatureTimestamp is known (the sts string) so a different body than the body present in the mocks is send by the extractor instance.
As a result, running all YouTube stream tests with the MockDownloader (like the CI does) will fail if this method is not called before fetching the page of a test.
Refactor all stream extractor tests to use new base class.
Remove check if upload date is in the past: this does not have to hold true: youtube premieres turn up in search results even though they are in the future