We have tests that hit multiple delta blocks for decompressFast(), here:

https://github.com/10gen/mongo/blob/ed7a17b021b69e4d9f41d036e31438c065e56f59/src/mongo/bson/util/bsoncolumn_blockbased_test.cpp#L567-L592

This code is not quite merged yet.

It would be good to also test at least one uncompressed literal in the scalar stream, since I don't think we hit that case yet. I think the elements that get passed into verifyDecompressPaths() could be chosen to force an uncompressed literal. Alternatively, we could build the BSONColumn manually using similar methods as used in BSONColumnTest.