byte algorithm in compression module: test coverage not satisfying

[Johannes Vollmer]

exrs/src/compression/mod.rs

Line 630 in 56f1d0a

 let source = vec![ 0, 1, 2, 7, 4, 5, 6, 7, 13, 9, 10 ]; 

this should test at least for 16 elements, because the algorithm is specialized for 16 elements

also it would be good to have a ground truth comparison instead of only a roundtrip test