Fix bug with caching in variables filter (with specs)

[simonneutert]

This PR attempts to finish the work from #106.

Please see my comment here: https://github.com/r18n/r18n-core/pull/106#issuecomment-2382594309

After formulating the problem, I came up with this possible solution. The spec counts whether duplicate keys are translated (the proper amount of times) or being loaded from the cache.

🚨 Current main branch runs thrice.

[codecov[bot]]

Codecov Report

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

Project coverage is 96.52%. Comparing base (dc3758d) to head (bb45cb7).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #111 +/- ## ========================================== + Coverage 96.50% 96.52% +0.01% ========================================== Files 83 83 Lines 2146 2156 +10 ========================================== + Hits 2071 2081 +10 Misses 75 75 ``` | [Flag](https://app.codecov.io/gh/r18n/r18n-core/pull/111/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=r18n) | Coverage Δ | | |---|---|---| | [](https://app.codecov.io/gh/r18n/r18n-core/pull/111/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=r18n) | `96.52% <100.00%> (+0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=r18n#carryforward-flags-in-the-pull-request-comment) to find out more.

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