give borgstore.Store a complete levels configuration, fixes #8432

[ThomasWaldmann]

also:

• add BORG_STORE_DATA_LEVELS=2 env var
• use BORG_STORE_DATA_LEVELS=0 to speed up tests

[codecov[bot]]

Codecov Report

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

Project coverage is 81.52%. Comparing base (f082df7) to head (0d269e7). Report is 3 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #8434 +/- ## ======================================= Coverage 81.52% 81.52% ======================================= Files 70 70 Lines 12797 12799 +2 Branches 2325 2325 ======================================= + Hits 10433 10435 +2 Misses 1705 1705 Partials 659 659 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.