Backport(v1.16) test: fix unstable buffer test (#4849)

[daipom]

Which issue(s) this PR fixes:

• Backport test: fix unstable buffer test #4849

What this PR does / why we need it:
Fix occasional test failure:
https://github.com/fluent/fluentd/actions/runs/13650069134/job/38156420830?pr=4846#step:6:4985

The max size of the buffer queue is 1 by default. So, try_flush will be called with flush_thread_interval, not flush_thread_burst_interval.

The previous test code assumes that the interval is 0.1s, but it was actually 1s.
This would be the cause of the occasional failure.

test: #write is called per value combination of variables, per flush_interval & chunk sizes, and buffer chunk is
purged(BufferedOutputTest::buffered output feature with variables):
            assert{ @i.buffer.stage.size == 0 }
                    |  |      |     |    |
                    |  |      |     |    false
                    |  |      |     1
                    |  |      {...}
                    |  #<...>
                    #<...>

/Users/runner/work/fluentd/fluentd/test/plugin/test_output_as_buffered.rb:1459:in `block (2 levels) in <class:BufferedOutputTest>'
         1456:       @i.enqueue_thread_wait
         1457:       @i.flush_thread_wakeup
         1458:
      => 1459:       assert{ @i.buffer.stage.size == 0 }
         1460:
         1461:       waiting(4) do
         1462:         Thread.pass until @i.write_count > 1

Docs Changes:
Not needed.

Release Note:
Not needed.