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GH-41558: [C++] Improve fixed_width_test_util.h  (#41575)



### Rationale for this change

Improve the `fixed_width_test_util.h`.

### What changes are included in this PR?

- Move the `fixed_width_test_util.h` to `arrow/testing`
- Divide the `fixed_width_test_util` to `.cc` and `.h`
- Remove unused headers

### Are these changes tested?

Yes

### Are there any user-facing changes?

No

* GitHub Issue: #41558

Authored-by: Hyunseok Seo <hsseo0501@gmail.com>
Signed-off-by: Felipe Oliveira Carvalho <felipekde@gmail.com>