GH-39326: [C++] Flaky DatasetWriterTestFixture.MaxRowsOneWriteBackpre…

…sure 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>

cpp/src/arrow/dataset/dataset_writer_test.cc

@@ -290,12 +290,12 @@ TEST_F(DatasetWriterTestFixture, MaxRowsOneWriteBackpresure) {
   write_options_.max_open_files = 2;
   write_options_.min_rows_per_group = kFileSizeLimit - 1;
   auto dataset_writer = MakeDatasetWriter(/*max_rows=*/kFileSizeLimit);
-  for (int i = 0; i < 20; ++i) {
-    dataset_writer->WriteRecordBatch(MakeBatch(kFileSizeLimit * 5), "");
+  for (int i = 0; i < 5; ++i) {
+    dataset_writer->WriteRecordBatch(MakeBatch(kFileSizeLimit * 2), "");
   }
   EndWriterChecked(dataset_writer.get());
   std::vector<ExpectedFile> expected_files;
-  for (int i = 0; i < 100; ++i) {
+  for (int i = 0; i < 10; ++i) {
     expected_files.emplace_back("testdir/chunk-" + std::to_string(i) + ".arrow",
                                 kFileSizeLimit * i, kFileSizeLimit);
   }