My recollection is that there were never any Battle Tests distributed during the lifetime of the NDS online services, so this content was never officially available for download & documentation.
As the years have passed, our understanding of the Gen5 save file structures has allowed us to identify the region of save file data that *would* have stored the downloaded battle test data, were it ever made available officially. However, since we don't have any of the official data, we can't really guess the size & layout of the data that needs to be there to get the game to recognize it.
With the power of disassembling the game code, we can see that the region of data is 0x5C8 bytes long, and the validation function checks for a magic number and checksum over the region being valid. When it's valid, you get....
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The latest commits of PKHeX allow for importing/exporting of this data region. This "DLC Battle Test" structure is undocumented. Fuzzing the rest of the data to values 01-04 gets me into a battle! Guess it has to be analyzed in more detail, but hey, at least we can trigger the in-game loading of injected data!
Attached a randomly-fuzzed download test that doesn't crash, which can be imported with the latest development build of PKHeX. Rather than disassembling the game code that interprets the block, we can just observe the results and see what offsets determine what, to get the overall structure documented
Battle Test.bt5