Tests need constant state

[ns-rse]

I've noticed whilst working on #65 that the regression test introduced in #62 failed.

As far as I could tell what had changed was the order of entries in the output, which is strange as the current code is looping over things so unless the order of the iterables changes each time code is run I wouldn't expect this to change.

For now though a temporary fix to ensure that the tests always pass is to sort the dataframe that has been introduced to undertake the regression test. This should be done based on the transcript_id, start and end columns.