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GH-41617: [C++][CMake] Fix ARROW_USE_BOOST detect condition (#41622)

### Rationale for this change

We also need Boost when ARROW_FLIGHT and ARROW_TESTING are true.

### What changes are included in this PR?

Add missing the condition.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* GitHub Issue: #41617

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