Problem:
Regression from de794f2d24: `vim.diagnostic.setqflist{open=true}` attempts to
open the location list instead of the diagnostics quickfix list if it didn't
exist before. This is because we are using `qf_id` to decide which to open, but
`qf_id=nil` when there is no existing diagnostics quickfix list with a given
title ("Diagnostics" by default).
Solution:
- Revert to using `loclist` to decide which to open.
- Add tests.
6c975515c5