Move test blobs to a submodule?

[njlr]

I am using the Buckaroo package for nlohmann/json and find it takes quite a long time to download. Looking at the repo, the test blobs seem to be the cause. Of course, these are not required when using the library.

Perhaps the tests should be moved to a Git submodule, so that they can be fetched optionally?

[nlohmann]

Duplicate: #482 #96 #620.