Add tests for identifying sg/sl and creating bysg/byperson datasets

[milanwiedemann]

This tests the functions involved in identifying sudden gains/losses and creating datasets (bysg, byperson) for further analysis.

This PR also fixes some minor issues (tidyselect) found when writing the tests.

[milanwiedemann]

@grahamthew this now adds tests for the following funs:

• identify_sg()
• identify_sg()
• create_bysg()
• create_byperson()