Fix filter splicing on expression

There was some bug with the id that was being used for the splicing data to get a common id to join with the expression ID. In addition, I added:

- Scrambled neural diff chr22 data, accessible as `flotilla._test_data` (other names are welcome, this is woefully uninformative to me)
- `test_study` fixture in `flotilla/test/conftest.py` which iterates over both the shalek2013 and the "`_test_data`", which is helpful to make sure some function works for both.
- Add test for `study.tidy_splicing_with_expression` with both shalek2013 and "`_test_data`"