Fix several `FileDB.from_disk()` issues

[gipert]

• find-and-replace logic was wrong
• case in which the columns already exist but with different index was not handled
• add tests based on new testdata: https://github.com/legend-exp/legend-testdata/pull/11

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.20 :tada:

Comparison is base (1c5c897) 48.22% compared to head (9243670) 48.42%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #495 +/- ## ========================================== + Coverage 48.22% 48.42% +0.20% ========================================== Files 104 104 Lines 12452 12460 +8 ========================================== + Hits 6005 6034 +29 + Misses 6447 6426 -21 ``` | [Impacted Files](https://app.codecov.io/gh/legend-exp/pygama/pull/495?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=legend-exp) | Coverage Δ | | |---|---|---| | [src/pygama/flow/file\_db.py](https://app.codecov.io/gh/legend-exp/pygama/pull/495?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=legend-exp#diff-c3JjL3B5Z2FtYS9mbG93L2ZpbGVfZGIucHk=) | `78.28% <100.00%> (+5.87%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/legend-exp/pygama/pull/495/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=legend-exp)

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