Group holiday tests into folders

[XSpielinbox]

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We are starting to approach a very large amount of holiday test files in the jollyday-tests/src/test/resources/holidays folder. To encourage people to create more such test files and allow for better overview it would be necessary in my opinion to move and sort them into sub-folders.

There the question arises:

1. Make one folder for each country and have all years of the country in that folder or
2. Make one folder for each year and have all countries for that year in that folder?

Whatever parameter is already in the folder name should not get duplicated into the file name in my opinion.