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MINOR: [C++] Use Cast() instead of CastTo() for Timestamp Scalar in test (#39226)



### Rationale for this change

Remove legacy code

This is a sub-PR of the PR mentioned below.

* #39060  

### What changes are included in this PR?

* Replace the legacy scalar `CastTo` implementation for Timestamp Scalar in test. It was supposed to be resolved in the mentioned PR, but it was missed. 

### Are these changes tested?

Yes. It is passed by existing test cases.

### Are there any user-facing changes?

No.

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