Replace `joinDatasets()` in `AbstractHtmlProcessorTest` with a method from `rawr/phpunit-data-provider`

[oliverklee]

[JakeQZ]

DataProvider::cross() "creates a matrix of given data providers". The joinDatasets method returns only the first row and the first column of that matrix. It is useful in situations where it is not necessary to test all combinations, but just ensure each of the input sets is included in at least one output combination, so as to avoid a large number of unnecessary tests. There is currently no equivalent in rawr/phpunit-data-provider.

[JakeQZ]

There is currently no equivalent in rawr/phpunit-data-provider.

DataProvider::zip() is the closest. If this was able to accept providers with differing numbers of rows, perhaps repeating the last row of the shorter ones (or cycling through), it would do the job. Alternatively, we could use DataProvider::join() to concatenate the shorter set with itself until it is long enough, then DataProvider.drop() to trim it to the exact length.