Split the helpers unit tests

This will be useful for the boundingBox and iiifUri refactoring (#738 (closed))

requested review from @babadie

assigned to @erwanrouchet

mentioned in merge request !1028 (merged)

mentioned in merge request !1029 (merged)

added 15 commits

• 38a65014...2f17800c - 14 commits from branch master
• ecfe8b10 - Split the helpers unit tests

Compare with previous version

enabled an automatic merge when the pipeline for ecfe8b10 succeeds

mentioned in commit 247b696e

merged