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GH-39049: [C++] Use Cast() instead of CastTo() for Dictionary Scalar in test (#39362)



### Rationale for this change

Remove legacy code

### What changes are included in this PR?

Replace the legacy scalar CastTo implementation for Dictionary Scalar in test.

### Are these changes tested?

Yes. It is passed by existing test cases.

### Are there any user-facing changes?

No.

* Closes: #39049

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