GH-46439: [C++] Remove unneeded namespace prefix in test_util_internal.h

[amoeba]

Rationale for this change

Addresses #46180 (comment). One of three PRs to resolve #46439.

What changes are included in this PR?

• Remove unneeded namespace prefix in test_util_internal.h.

Are these changes tested?

Yes. Impacted tests still pass.

Are there any user-facing changes?

No.

• GitHub Issue: [C++] Address post-merge review comments in PR exposing {Array,...}FromJSON helpers in public API #46439

[kou]

+1

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 4 benchmarking runs that have been run so far on merge-commit 58b8a6f.

There were 75 benchmark results with an error:

• Commit Run on arm64-t4g-2xlarge-linux at 2025-06-04 03:19:11Z
  • tpch (R) with engine=arrow, format=parquet, language=R, memory_map=False, query_id=TPCH-07, scale_factor=1
  • tpch (R) with engine=arrow, format=native, language=R, memory_map=False, query_id=TPCH-09, scale_factor=1
• and 73 more (see the report linked below)

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 4 possible false positives for unstable benchmarks that are known to sometimes produce them.