Avoid using ComparisonUtils::compareXml in CI

[flo-dup]

Describe the current behavior

ComparisonUtils::compareXml is used in unit tests

Describe the expected behavior

ComparisonUtils::compareXml is not used in the CI to reduce the corresponding time cost. ComparisonUtils::compareTxt or Files::mismatch used instead

Describe the motivation

Reduce time spent in CI

Extra Information

With the TreeDataReader/Writer and JSON format we now enforce a precise attribute order in reading/writing. Hence we could use compareTxt at the cost of updating many (if not all) reference unit tests. We might go further and use the byte-by-byte comparison instead, but the line separators would probably cause an issue on Windows CI.