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GH-39326: [C++] Flaky DatasetWriterTestFixture.MaxRowsOneWriteBackpresure test (#39379)



### Rationale for this change

This patch reduce the number of open files in testing first. I've verify the test in 14.0.2, it hangs forever.

### What changes are included in this PR?

Change the test file number from 100 to 20

### Are these changes tested?

Already

### Are there any user-facing changes?

no

* Closes: #39326

Authored-by: mwish <maplewish117@gmail.com>
Signed-off-by: mwish <maplewish117@gmail.com>