arrow

GH-42005: [Java][Integration][CI] Fix ARROW_BUILD_ROOT Path to find pom.xml (#42008)



### Rationale for this change

This PR aims to fix the issue where the integration tests are failing due to the missing `/java/pom.xml` file. It appears that the current code incorrectly determines the path to `ARROW_BUILD_ROOT`, leading to the failure in locating the `pom.xml` file.

### What changes are included in this PR?

- Updating the `ARROW_BUILD_ROOT` path determination logic in `tester_java.py` to correctly reference the project root.

### Are these changes tested?

Maybe, Yes.

### Are there any user-facing changes?

No.

* GitHub Issue: #42005

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