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GH-39050: [C++] Use Cast() instead of CastTo() for Timestamp Scalar in test (#39060)



### Rationale for this change

Remove legacy code

### What changes are included in this PR?

* Replace the legacy scalar CastTo implementation for Timestamp Scalar in test. The cast function already supports casting between different timestamp types. We just need to allow for the possibility of data loss by using `CastOptions::Unsafe()`. 

### Are these changes tested?

Yes. It is passed by existing test cases.

### Are there any user-facing changes?

No.

* Closes: #39050

Authored-by: Hyunseok Seo <hsseo0501@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>