Increase test coverage in `saving`

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: -0.19% :warning:

Comparison is base (74a4e7f) 76.78% compared to head (8a40bac) 76.59%.

:exclamation: Current head 8a40bac differs from pull request most recent head 22a8123. Consider uploading reports for the commit 22a8123 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #895 +/- ## ========================================== - Coverage 76.78% 76.59% -0.19% ========================================== Files 329 329 Lines 31431 31429 -2 Branches 6113 6114 +1 ========================================== - Hits 24134 24073 -61 - Misses 5726 5778 +52 - Partials 1571 1578 +7 ``` | [Flag](https://app.codecov.io/gh/keras-team/keras-core/pull/895/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team) | Coverage Δ | | |---|---|---| | [keras_core](https://app.codecov.io/gh/keras-team/keras-core/pull/895/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team) | `76.49% <ø> (-0.19%)` | :arrow_down: | 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=keras-team#carryforward-flags-in-the-pull-request-comment) to find out more. [see 10 files with indirect coverage changes](https://app.codecov.io/gh/keras-team/keras-core/pull/895/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team)

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