Write integration tests for src/acquisition/covidcast/database.py

Integration tests because we do want to exercise the actual database engine here, and not a mock.

• make sure is_latest_issue is being set correctly -
  • when adding new daily data (data in signal_load has > issue than data in signal_latest)
  • when uploading data patches (data in signal load has < issue than data in signal_latest)
• make sure data is being correctly transferred from signal_load into signal_* and *_dim
  • make sure signal_latest is only overwritten when signal_load issue is >=
  • make sure rows are added to *_dim only when needed

+Any other corner cases that worry you.

Use integrations/acquisition/covidcast/test_*.py as templates.