Only run integration tests once, not in each environment

[mesozoic]

This will make tox a lot faster on local machines and seems very unlikely to miss any regressions.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (cfd304b) to head (ba870cd).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #359 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 25 25 Lines 2315 2315 ========================================= Hits 2315 2315 ```

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