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GH-36331: [C++][CI] Sporadic errors in AsofJoinTest (#36356)

### Rationale for this change

Skipping a flaky test until a robust way for it is found.

### What changes are included in this PR?

The `AsofJoinTest.BackpressureWithBatchesGen` test is skipped.

### Are these changes tested?

N/A

### Are there any user-facing changes?

No.

* Closes: #36331

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