Add tests for the UpDropEventReferenceSHAPrevEvents migration

[S7evinK]

... as they could fail if there are duplicate events in roomserver_previous_events.
This fixes the migration by trying to combine the event_nids if possible (same room) as mentioned by @kegsay in #3083 (comment)

[codecov]

Codecov Report

Patch coverage: 45.94% and project coverage change: -0.12 ⚠️

Comparison is base (3dcca40) 66.36% compared to head (02ff5ab) 66.25%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3087      +/-   ##
==========================================
- Coverage   66.36%   66.25%   -0.12%     
==========================================
  Files         499      499              
  Lines       53809    53908      +99     
==========================================
+ Hits        35710    35715       +5     
- Misses      14545    14600      +55     
- Partials     3554     3593      +39     

Flag	Coverage Δ
unittests	49.96% <45.94%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...stgres/deltas/20230516154000_drop_reference_sha.go	43.24% <45.45%> (+5.31%)	⬆️
...qlite3/deltas/20230516154000_drop_reference_sha.go	40.62% <46.42%> (+40.62%)	⬆️

... and 18 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[IngwiePhoenix]

Any chance this is merged soon? Been rolling around in the commit history to find a good one to use to temporarily fix #3091 ...