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GH-43040: [C++] Reduce the recursion of many-join test (#43042)



### Rationale for this change

The current recursion 64 in many-join test is too aggressive so stack (the C program stack) overflow may happen on alpine or emscripten causing issues like #43040 .

### What changes are included in this PR?

Reduce the recursion to 16, which is strong enough for the purpose of #41335 which introduced this test.

### Are these changes tested?

Change is test.

### Are there any user-facing changes?

None.

* GitHub Issue: #43040

Authored-by: Ruoxi Sun <zanmato1984@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>