test(partitions): gate the persist-retry test on Linux

[hubcio]

The retry test reaches for /dev/full, which only exists on
Linux, but its #[cfg(target_os = "linux")] gate and doc
comment landed on the preceding test instead. macOS builds
of the test target failed with E0425 on DEV_FULL.

Pre-merge builds macOS without --tests, so the break only
surfaced in the post-merge macOS cache warm, which does
compile tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.90%. Comparing base (8b7dbd9) to head (30fccc3).

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3998       +/-   ##
=============================================
- Coverage     84.91%   73.90%   -11.02%     
- Complexity     1405     1406        +1     
=============================================
  Files          1224     1224               
  Lines        179327   179327               
  Branches     145615   145616        +1     
=============================================
- Hits         152271   132527    -19744     
- Misses        23028    42737    +19709     
- Partials       4028     4063       +35     

Components	Coverage Δ
Rust Core	72.16% <ø> (-13.64%)	⬇️
Java SDK	67.40% <ø> (+0.05%)	⬆️
C# SDK	75.39% <ø> (ø)
Python SDK	90.06% <ø> (ø)
PHP SDK	85.65% <ø> (ø)
Node SDK	94.51% <ø> (-1.63%)	⬇️
Go SDK	69.36% <ø> (+0.06%)	⬆️

Files with missing lines	Coverage Δ
core/partitions/src/iggy_partition.rs	76.70% <ø> (-12.90%)	⬇️

... and 355 files with indirect coverage changes

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[djanand]

Looks good. Facing the same on ci-pre hooks when merging upstream/master to forked repo.