GH-34269: [C++] Fix include file name (#34285)



### Rationale for this change

I got a "arrow/dataset/test_util.h: No such file or directory" error when building, so I changed it to the correct file name.

### What changes are included in this PR?

Changed include file name.

### Are these changes tested?

I checked that I was able to build because I had an error in the build.

### Are there any user-facing changes?

* Closes: #34269

Authored-by: abetomo <abe@enzou.tokyo>
Signed-off-by: Weston Pace <weston.pace@gmail.com>